Oh happy day!
I had been keeping an eye on the weather and saw that it was supposed to be 45 degrees today, so I decided that that would be the day that I would fire up my compressor again and let it run full-up Nitrox and see how she's doing.
Seeing as I had a few days to wait, and in addition I had something like 16 tanks sitting in my garage (which is not where my compressor is),
I finally ran steel tubing from my compressor to the front of my garage -- no more dragging tanks around just to fill them! So today, got the compressor prepped, I hooked up a set of tanks in my garage, and hit the button. The tanks I started with were at 1200 PSI, so the compressor spent a bit of time at 2000 (at the PMV pressure), then built up all the way to 4200 PSI. No rattle, no clunks, nothing but the rhythmic purr of a well-tuned compressor. OK, it's quite a bit louder than a purr, but it was steady and regular, which is what I cared most about!
I ran it for just over 2 hours today. I filled two sets of doubles, and brought up one of my bank bottles to full pressure. No variation in the sound throughout. From my previous run, I thought there might have been a bit of an issue between 2000 and 3000 PSI, but I ran it through that area three times today and there was no change. So I think I'm ready to call this sorted.
I've also decided to
send my auto-drain off to Vermont to get repaired -- I'll update that thread with more details. If I can get that back in place, the compressor will be better than it's ever been since I've owned it.
Once again, thank you all very much for your help! I really appreciate it.