Wondering if anyone ever tried using nylon bolts and nuts on the PT? Would ground be an issue?
Reason I ask, is there's still metal to metal contact between the chassis and transformers as there's only one isolation washer per bolt. I added some domed faucet washers when I built mine - never had it without, so not sure if that helped or not. The washers I used were undersized and drilled to match the bolt diameter which allowed them to slip into slightly oversized chassis holes - that also centered the bolts so those can't touch the chassis plate. Be a bit difficult to do that after the build though.
Another thought - get some hard rubber flat washers and cut a notch so you can slide them in between the PT mounting tabs and chassis on the top after you loosen the bolts. A wrap or two of black tape on the bolt itself where it would otherwise be able to go metal to metal on the chassis should help isolate things and reduce any residual hummmmmmmmmm ...
Still ... when all is said and done, it's not likely that little bit is gonna matter at any listening level. I know mine has a slight mechanical noise to it, but I can't say as I've ever noticed it even during ultra black passages on uber good source.
PS ... an example of how getting too close can drive you to distraction. I just replaced the water pump on my car the other day. That cured the little bit of a rattle that had been bothering me, but now I can hear a slight whine coming out of the power steering pump. Pointed that out to a neighbor when he stopped by while I was buttoning things up, who of course said "Noise" What noise?"