Noisy 70's fuel pump?

[TJ Hopland]

This is on my 73 Eldo that I rebuilt the engine in a several years ago.  When I first rebuilt it I ran it a few hundred miles with a carb and likely the original pump.   Converted to an EFI system with an inline pump so I remove the mechanical pump.   Couple years ago now went to a different efi system where the high pressure pump is in a little tank that is fed by the mechanical pump.   Since then I have had a odd cam speed related noise that is most easily heard from sitting in the front seat.  I'm on my 3rd pump now and they all have same but different noise.   Annoyingly I have bought 3 different brand new pumps and they were all identical and make similar noises.   Different boxes and labels but the exact same pumps inside.   

I don't think the eccentric is loose but only checking it so far by poking a screwdriver through the pump slot.  I'm not sure if I am just dealing with crap quality pumps or I have some other problem.   So far what I have been using is the original style for a 73 with the big canister and the internal filter.   One thing I have not yet tested that may be slightly unusual is I have the return port caped.  The return in my system is from the canister that has the electric pump.  Does anyone think having the return capped could be causing the noise?  Seems like people cap them fairly often and don't talk about noise.   Just to see I was going to try and run a line from that into a container and see if the noise quits or changes.     

Any other ideas?   I have tired adjusting it all the way up and down within the slop of the mounting holes.   There seems to be spring pressure on the lever even when the eccentric seems to be all the way up.     

[Jeff Rosansky CLC #28373]

Are you able to swap pumps with a known good one on another car?
Just thinking out loud here but if the mechanical feeds a header tank maybe there is no pressure restriction so the pump is just able to flop around with no resistance. Pressure in the output line may be necessary such as resistance in an electrical circuit.
If you can swap out the pump on another engine and the sound goes away/the known quiet pump turns noisy that may confirm my thinking.
Jeff

[TJ Hopland]

I don't currently have another running engine from that family or know anyone close by that has a running one.   Good thought on maybe not enough back pressure.   It does sound like something just doesn't have enough load on it.

[Greg C]

put a hose on the return line stick it in a pail and run it

[TJ Hopland]

My plan at the moment is to pinch off the return and see if that changes anything.  If no change then with that still pinched I will try connecting a hose to the pumps return into a container and see if that changes.  I could also try un pinching the return with the pump connected to the bucket. 

Does anyone know how the return in the pump works?   Does it always bypass some fuel so its just a restriction?  Or does it have a pop it valve on it so that at say 3 psi it opens?   What is in all the extra space in the Cadillac ones?  It does seem like most of the space ends up with fuel in it.  Even the 425 ones with no filter had that extra space in it.  I now have 3 pumps so I guess I could cut one open and see.