Doc, the most common causes are grounding and electrical interference on the cables in the amp from other components in your set up or even other components within the amp. Does the amp give out the noise even when all input cables are pulled out and only the speakers are connected? For starters, you could try:
1.Connect the amp power cord of the amp to the same power outlet/wall socket distribution board as your other components that may be connected to the amp, like preamp, source of music, etc.This should eliminate any ground loop.
2. Ensure that the signal cables to and from the amp do not go very close to the power board or power cables.
3. Try opening the amp cover and use a small blower or vacuum cleaner to blow away dust and cobwebs, if any. Then try pushing aside the signal cables away from the transformer, carefully so that all connections remain intact.
4. If the above steps don't help, get an electronics guy to check the sockets on the amp
5. Search the net... there are many discussions on several forums on this subject.
My Cary power amp had this issue which was totally eliminated by pushing aside the cables inside the amp.
Hope you are able to resolve this.
Cheers!