Buzz coming from power amp - not through speakers

Hi, first post here. I just picked up a Harmon Kardon PA5800 off ebay. It sounds great, no issues there. However, the amp itself makes an intermittent hum/buzzing noise (ie, the sound comes out of the amp unit, not through the speakers). It fades in, peaks, then fades back out with a period of about 5-7 seconds. Sometimes it's not noticeable at all, but occasionally it's loud enough to hear even with a moderate volume level through the speakers.

Anyone have ideas? DC noise on the AC line? Basic service needed, ie new filter caps? (I'm a tube guitar amp guy, not even sure this type of amp has filter caps).
If you are using it with a TV, then you very possibly have a ground loop problem related to your cable or satellite box.
I use the L & R audio outputs from my satellite box to feed my receiver and not the outputs from the TV. I just mute the TV speakers.
Yep, I that's what I was doing. RCA outs from the directv box to a Marantz AV7005 and into the H/K, with the TV speakers set to zero. I would be running everything HDMI through the Marantz but I don't have cables long enough yet. I'm going to be rearranging the whole room over the next couple weeks, so it's possible that a new outlet and/or different location in the room could clear it up.
I may be interferece getting on your power lines . The transformer in that amp is apparently more susceptible to it than other amps. Check for any flourescent lights or dimmers being used that use the same circuit. It could be getting worse each time your refrigerator or heating unit kicks on too. Try to isolate the amp circuit from the digital and tv circuits.
I had a very high end integrated that had a similar "periodic" noise and the solution ultimately was teh manufacturer redesigned the transformer. In this case, it is not an option so the "preventive" measures are to attempt to minimize the noise to a tolerable level.

A power conditioner may help but my experience is it doesn't. I had some success with a PS Audio humbuster but not always. The humbuster is good at stopping DC noise but not much else and usually it is not "just DC noise" you are getting.

The gentleman who recommended "tack" I believe was referring to "Blu-tac" which is an expensive version of mortite (also available at places like HD or Lowes, much cheaper and just as effective). Basically you would use it to damp the transformer to tame the vibrations. With a toroidal transformer it is usually the internal windings that are vibrating so the noise will not stop but the damping may at least reduce the "volume".
All transformers buzz and the changes you are hearing may be related to power line harmonics. HTH is correct about the windings vibrating not the core. If you own a smartphone, download a microphone app that records and displays the audible frequency; a number of them exist for very little $$. I'm guessing if it's 60 HZ it's mains related, if it's 120HZ or higher it may be power supply noise on the secondary winding of the transformer. In either case, it's likely not out of the ordinary.
Hi, I don't know if you solved your issue but a while back I went through some similar problems with my Classe CT-2300 and to a lessor extent an Acurus A200 used for surround channels.
Some phone time with Classe tech support and through some experimenting, it was decided that my Plasma television was introducing DC into the AC line.
I have since plugged the television into a different circuit altogether, and my complete audio system is on two circuits on the same power leg.
My amplifiers are quiet and my system sounds great.