Adcom GFP 750 preamp "hum"

Hello, I have had the 750 for around ten years and it works like a charm. I bought it new. However, last weekend I was leaning over dusting off some gear and as my head got closer to the preamp I heard a slight hum. When I stuck my ear withing an inch of the unit, it was definitely a hum being generated from something within the unit.

Everything was off, no gear was on. The preamp was in "stand by" mode (as far as I know, this allows a trickle of power to the preamp to "keep it warm".) There were no other major appliances on drawing from my electrical service.

Then, when I checked it out later that evening, the hum was gone! Still in standby mode, still nothing else in the system was on, but the hum was gone! I don't use a power conditioner.

Could the utility juice to the house be fluctuating! Is anything being harmed by the "hum"?

Any input would be appreciated.
Best Regards,
It is probably the torroidal transformer humming because of DC on the AC line--and that could explain why you only hear it certain times of day. I wouldn't worry about it if there are no ill effects during listening.

I owned a GFP750 a few years back, and I think mine hummed a bit.
No problem.
agree with Swanny76109
Thanks Swanny and Elizabeth. I'll disregard it. It's actually been very quiet lately.