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,
I had a Belles 150a Hot Rod that annoyed me mightily with the same problem. The toroid transformer in these amps are very easily excited by noise pollution in the ac lines. I found that when fans or certain ac appliances would turn on in the house (not on the same circuit) the amp's toroid would start humming. If your hum is intermittent, you might see if there is any correlation between appliances/ac/etc. operation and the hum. Perhaps ac power treatment would help, or maybe you just have an amp that's going to hum. When I talked to Dave Belles, the owner of the company and designer, he basically offered no help and admitted that the amp was prone to this peculiarity.
Photon46 is right on the mark. I have a Belles 150a hotrod which also had an intermitant transformer hum which I eventually tracked down to a humidifier on the other side of my apartment. Took me forever to isolate this. Before I figured this out I did try an AC power conditioner which solved the problem although I did feel it constricted the amp somewhat. I should add, hum issue solved, that I think this is a great amplifier. Good luck!
As mentioned, many appliances set off the humming. Even though I have dedicated lines from a separate sub-panel, I still hear the hum whenever the microwave is used. Other motors and appliances do the same. Worst case, you may need to go to a line conditioner of some sort -- preferably one that attacks both small amounts of dc voltage that could be in the line (you could check that) and the spurious high frequency noise that is part of our AC system.
I've got to agree. Mine hums if the dryer is running (I told my wife I'd beat her if she does THAT again!), microwave, etc.... I don't like to use a power conditioner with this amp. It normally sees duty as a sub amp, and the hum is never noticable there.

But, now I've got it hooked up in my main stereo system so I can track down a problem with my Audio Note amp. Mated to the Supratek Syrah with 93db AN speakers, any noise on the amp end comes through.

Admittedly, it's a pain in the A@#, but I do like the amp. So, I just have to be careful to schedule my listening time when the old lady's not home to turn shit on and annoy me! FWIW