"This is one part I wouldn't swap without full diagnosis"

Amen, Art.

Glad of the help, sometimes all you need is another knowledgable person to bounce ideas off of.

I checked the pre-pump current draw at the fuse as one of the manuals said to and it was drawing a little over 5 amps. Now according to the book it should have been 3-4 (values for the newer pump which the dealer would have replaced it with). The way I figure, the extra current draw could be because of a failing pre-pump, a clog that the pump(s) is working against, or a main pump not doing it's job, thus making the pre-pump work too hard. Dad, a life-long electrician also says that insufficient voltage could also be a cause, and voltage checked at the pump was, I believe, something like 10.5v - a little low, yes?

The rubber pump mounts seem in good shape to me and I've also tried to still the pump with my hand, hoping that it was indeed just some errant vibration.

Tomorrow's plans include removing the check valve and pump and giving them a visual check to see if there's any obvious obstruction. Possibly I'll drop the pump into a bucket of oil or gas and run it with a power supply to check for the noise - think that's a good plan?

I suppose an electrical check of the main pump and voltage at the pump is in order too. I assume Bentley will have the expected values.

Thanks again, I hope to get this cleared up soon so I can hear the sweet note of a brand new exhaust.

-Sean