Constrained layer damping tape on toroidal transformer?

No.

You could insulate the transformer, causing it to heat up, and the conductor itself could have currents forced in it, causing it to heat and melt.

If you are trying to reduce physical hum, better to look at the mounting method and eliminate DC on the AC line.

Best,
E

Glad I asked! 

Place the viscoelastic material under the transformer. Thus it will be constrained by the transformer. Loosen or remove bolts to isolate it from the chassis. Problem solved! 🤗

@geoffkait is right, but don't use anything conductive!