The output transformers will produce a small sound, especially if not connecting load.
Even output transistors in solid state amp’s tend to “sing” along with the signal, though the mechanism is different. Capacitors (e.g. in Speaker Crossovers) often have a similar issue.
All electronic components are subject to microphonics and with signal applied reverse microphonics. If the sound output is material depends on many factors.
The job of the transformer is to transfer power into a load. Without load the power has no-where to go. This causes electrical and mechanical resonances in the transformer to be “fed” with spare power which would normally be damped by the load.
This will make microphonics worse and further can create very high voltages (resonance – no load), even high enough to create a miniature lighting strike inside the transformer (which usually burns away the isolation and renders the transformer defective).
Modern isolation such as we use at iFi should handle this, but better not walk on the wild side.
So a few rules here:
1) Never operate ANY tube output amp with output transformers without load.
2) At high playback levels some “singing along” noise from power amplifiers is expected.
