Variacs are not built with the intention to be particularly quiet. The only one that was to my understanding was made by MacIntosh back in the 1960s.
Also something to be aware of with any isolation transformer- 50% of rated capacity is a safe limit. The transformer can make significant distortion if loaded above 50%. That distortion can cause diodes and transformers downstream to become noisy.
I think Erik might be on to something. If you have a ground loop, its not uncommon for an isolation transformer to fix it. But its usually better if you find what is causing the ground loop and fix that. Even if they don't hum or buzz, ground loops degrade the sound overall by increasing intermodulations.
Also something to be aware of with any isolation transformer- 50% of rated capacity is a safe limit. The transformer can make significant distortion if loaded above 50%. That distortion can cause diodes and transformers downstream to become noisy.
I think Erik might be on to something. If you have a ground loop, its not uncommon for an isolation transformer to fix it. But its usually better if you find what is causing the ground loop and fix that. Even if they don't hum or buzz, ground loops degrade the sound overall by increasing intermodulations.