Their are capacitor banks that stay charged whe unplugged. If you unplug and turn on amps it will bleed them down. I always temp install lid if I am going to power it up. A teenager in household. The removed lid wrapping on sheet metal turned over you will hear wear it sing. That is where to concentrate on dampting materials.
I removed the lid and definitely found that the buzz noise is coming from the toroidal transformer. See pics attached.
I firmly strengthened the toroidal bolt and reseated back the lid, then I better strengthened also the screws on the lateral panels in order to reduce any possible vibration. Two of them weren’t completely fasten.
The noise isn’t vanished but is acceptable at the moment, let me listen to music for a while to confirm my impressions.
Then I removed also from the P20 the IsoAcoustics Orea feet and next weekend I’ll do the same work on its toroidal bolt.
At the end I put a 3M absorber sheet under both the P20 and P15, not for sound improvement but to find out if it can mitigate more the magnetic field near the transformer.
Thank you for your help, hope I can live with the actual subtle noise, that is far less than before.
I’d try two half moon Sorbothane sheets under toroidal you only have to loosen enough to get under both sides. I am glad you got some relief. But I bet you can damp it out completely.