Variac noise

Hi all. Not sure if this is the correct place for this question but here goes…

I have a Marantz model 8 from Japan running at 100volts through a variac. The variac hums while under load and it’s (just) loud enough to be annoying. I have a friend who uses the same variac for a similar setup and his is dead silent. Thought it might be a bad variac so I ordered another and set it up and still the same problem, so could it still be the variac? It’s a brand sold through Amazon and quite a few buyers complain about quality control problems with this product. I’ve looked at buying an American made variac but they cost more than $700. Could the amp be causing the variac to hum?

Any thoughts?