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,