1970s Worn Motor Mount Chattering or Something Else?

[SaltLife97]

Before I decide to drive to the mechanic or have it towed, am I correct to assume this is an original worn motor mount chatter? This sound only appears when putting the car into gear with the foot on the brake, so when the engine is put under load it drops idle speed and naturally vibrates more. Putting the car in park or driving it causes the engine to smooth out and not vibrate thus the sound disappears. I can't seem to pinpoint it with a stethoscope and the knocking sounds like it correlates more to the vibration of the engine vs being consistent with the rpm.

You can also sort of feel it tapping through the steering column/gear selector. It's driving me nuts. I already shimmed the new exhaust piping because it touched the frame so I stuck a flat piece of steel in between to fill the gap. I also shimmed a gap between it and the torsion bar with a fiberglass piece of cloth so I don't think it is that either.

https://www.dropbox.com/scl/fi/eexqxhw07050e553ub8h8/Cadillac.mp4?rlkey=0hffi3y5tjlqj2v2r9go8j1mt&dl=0

[signart]

Doesn't match your description, but from your video, the sound is exactly like an exhaust leak. for one.

Secondly, it also gives the sound of a jumping spark, from a plug wire or such. This would match your description, but this opinion is based on watching the video and actual diagnosis may vary.   

[Lexi]

To check for a worn motor mount do the so-called Power Brake test. Start car, apply the brakes, put the car in drive & gently apply the accelerator while an assistant looks at the motor with the hood up. The engine will tilt to the worn mount side, aggravated by the fact that the car can't move as the brakes are applied. Do in both Drive and Reverse and be careful not to apply too much accelerator pedal when completing this test. Clay/Lexi

[TJ Hopland]

You had places the exhaust was touching things so you jammed solid steel in there to fix it?  All thats doing is coupling the vibrations directly in those locations.  Thats probably why it sounds so strange since the exhaust normally isn't tied to the frame/body that close to the engine.

If it was a factory exhaust then issues with the mounts could cause the issue.  With a non factory system its hard to tell.  If it was made and fit with bad mounts or the engine is just able to move too much that is just going to make things worse.

What year is this?  73-78 used fairly typical mounts (possibly used by other GM stuff in the era) on the engine and a single off the back of the transmission.  Earlier had a single mount at the front of the engine and 2 at the back corners of the trans.  I think this earlier design is very hard to find now and possibly only option is sending yours in to be rebuilt.  The later I think you can get the engine ones but not the transmission one.   

[The Tassie Devil(le)]

The engine mounts are on either side of the pressed steel cover at the back of the transmission chain cover, and attach to each side of the chassis.   If it is a '72 engine, then there is only a single mount at the front of the engine, under the front of the sump.

That noise is not from engine mounts from what I can hear, but being localised at the left rear, it does appear to be a metallic sound.

There is not much room for the exhaust pipe to travel down the back, and being a dual exhaust, everything has to be clear of any obstruction.   When I built my own dual exhaust, the pipe came very close to the torsion bar.

Feeling the tapping through the steering column indicates that something is too close.   

Hard to diagnose over the internet.

Bruce.

[TJ Hopland]

I / we were not saying failed mounts would be directly causing that sound but that a failed mount could have let the engine move enough that exhaust is now hitting somewhere.

If there wasn't mention of exhaust clearance issues I would maybe be looking elsewhere but since it sounds like an exhaust rattle and there was talk about jamming steel plates between things that's where I'm staying focused.

One extra thing to consider when looking for this and or fitting a system is that the car has is being supported by its wheels.  If its on ramps or a jack there is going to be some to a lot of flex in the frame as well as things like torsion bars don't just twist in a straight line.

[SaltLife97]

You had places the exhaust was touching things so you jammed solid steel in there to fix it?  All thats doing is coupling the vibrations directly in those locations.  Thats probably why it sounds so strange since the exhaust normally isn't tied to the frame/body that close to the engine.

If it was a factory exhaust then issues with the mounts could cause the issue.  With a non factory system its hard to tell.  If it was made and fit with bad mounts or the engine is just able to move too much that is just going to make things worse.

What year is this?  73-78 used fairly typical mounts (possibly used by other GM stuff in the era) on the engine and a single off the back of the transmission.  Earlier had a single mount at the front of the engine and 2 at the back corners of the trans.  I think this earlier design is very hard to find now and possibly only option is sending yours in to be rebuilt.  The later I think you can get the engine ones but not the transmission one.   

The vehicle is a 1973 Eldorado. The exhaust was custom done and shipped across the U.S but alas the bending/cutting was not done THAT precise and only the driver side downpipe going to the Y Pipe was just barely clearing the frame on the driver side. As such, during low rpm under load conditions, the exhaust would definitely clatter against it and you'd feel it under your left foot. I machined a piece of steel wrapped in fiberglass cloth thick enough to be shimmed in using a pry bar. So it is TIGHT but I can try to verify this by removing it, replicating the situation, and prying the exhaust with a pry bar to see if giving more clearance is the solution to the knock.

Being honest, I also had an ever so clearance between the driver-side torsion bar and the exhaust pipe. I did put fiberglass insulation between the two as well at the time and it muted THAT soft knock but this one sounds newer and louder. Ideally I'd have a shop fix the exhaust and as time permits of course I'll make it right. I'm taking it to my trusted mechanic anyway I just don't want to be driving to it with something serious as a broken/cracked flexplate or something.

[SaltLife97]

There is not much room for the exhaust pipe to travel down the back, and being a dual exhaust, everything has to be clear of any obstruction.   When I built my own dual exhaust, the pipe came very close to the torsion bar.

Feeling the tapping through the steering column indicates that something is too close.   

Hard to diagnose over the internet.

Bruce.

I agree with something being too close especially since these noises began AFTER I installed the new exhaust system. I thought shimming it with fiberglass covering steel would help some but I could be chasing my tail on the exhaust transmitting knocks. It's tough because using a stethoscope while inhaling the hot fumes and not being able to pinpoint the sound is pissin me off. The valvetrain for example sound like normal clicking valves. And the transmission bell is ever so slightly emanating the knocking noise I hear but it is super muffled so I'm thinking something really is tapping against metal.

[SaltLife97]

Doesn't match your description, but from your video, the sound is exactly like an exhaust leak. for one.

Secondly, it also gives the sound of a jumping spark, from a plug wire or such. This would match your description, but this opinion is based on watching the video and actual diagnosis may vary.   

I would hope it isn't jumping spark since the wires are brand new but that is something I might be able to isolate and check. All I hope is that making a trip to the shop won't blow a rod though the block if that knock is internal

[The Tassie Devil(le)]

Here are some pictures from my own exhaust under my '72 Eldo. 

The exhaust ring under the left engine pipe is to maintain clearance whilst building.

The centre pipes are massaged to gain clearance under the torsion bar crossmember, and both tail pipes go out the one side, as the fuel tank is offset to the left.  Didn't have sufficient room to fit resonators, but they might come later.

I utilised multiple flanges for joining as this made manufacture a lot easier.  The system in heavy wall Stainless.

Since doing the exhaust, I have fitted a custom Sway Bar, so even tighter clearances.

Bruce.

[SaltLife97]

Here are some pictures from my own exhaust under my '72 Eldo. 

The exhaust ring under the left engine pipe is to maintain clearance whilst building.

The centre pipes are massaged to gain clearance under the torsion bar crossmember, and both tail pipes go out the one side, as the fuel tank is offset to the left.  Didn't have sufficient room to fit resonators, but they might come later.

I utilised multiple flanges for joining as this made manufacture a lot easier.  The system in heavy wall Stainless.

Since doing the exhaust, I have fitted a custom Sway Bar, so even tighter clearances.

Bruce.

Nice setup, How was your experience getting the driver side pipe OVER the torsion bar? Mine has a gap about an inch or so and knocks against it if I don't put that fiber glass insulation between it? I'd hate to have it rebent

[TJ Hopland]

There were changes to the frame between 72 and 73 related to the impact bumpers and it seems like since they had to make those changes they made some other tweaks while they were at it like flipping the mounts.  I never had both styles side by side to compare where they changed stuff.