Dealing with transformer hum

My Belles 150 makes a humming/vibrating sound which I'm told is due to the transformer. At average listening levels, the hum is not an issue at all, because it's not loud enough to be heard, but I do a lot of really low level listening at nights, and this hum is driving me absolutely nuts.

I don't have the option of really moving the amp much and there are no enclosures to hide it away in. Any ideas?

Thanks in advance,