update: The noise has something to do with the fuel return line surging into the tank. I was under the car today and checked the flex plate. It looked okay to me, though there wasn't too much room to see. I ended up replacing all of my fuel lines in the "going crazy" process as a preventative measure (and maybe trying to get the power back in the engine?). I also threw some sea foam in diesel-purge style from a can and the noise stopped. However, I noticed the return line was putt-putting pretty bad, though I think the cigar hose is designed as sort of a muffler for this. I was happy to realize the noise stopped, but now I wonder if this is normal for the lift pump to be so violently pumping that it creates an audible noise in the tank?
FWIW the fuel is clear finally (after draining the tank, I have another thread on this) and replacing every little bit of fuel hose under the hood seems to have sealed things up and restored some power. I'll admit I freaked out a bit on the flex plates
