To elaborate on Matt's last post, it is possible there is a "noise generator" within the house or nearby.
One trick is to use a battery operated radio capable of receiving the AM radio band, and tuning that radio down to the very low end of the AM band....in the range of say 540 to 640 kHz....where there are no local broadcasts to be heard. You will probably hear some "racket", but if you hear a loud screaming buzz across the unused frequencies, there is a local noise source.
Years ago, I made a service call to a studio which had been in operation for years and they began hearing a low, but obnoxious, buzz in multiple pieces of gear. The obnoxious buzz would begin ramping up in volume, then suddenly quit, only to repeat again, almost like clock work.
I went to my car in the parking lot, and tuned the radio to...maybe 600 kHz AM...an unused channel here. I heard the same "noise pattern"!
Walking around that studio building, I discovered a (mercury vapor?) security light on the building that was struggling to illuminate, even though it was broad daylight. Apparently the photocell had failed AND the lamp/ballast/whatever was also defective. Every 30 seconds or so, the security lamp "struggled" to light-up, then abruptly shut off.
That was acting as a noise generator which radiated into the audio system. As a quick fix, I disconnected the security light from the AC mains, and told the studio's manager to call an electrician to repair the fixture.
Bottom line...the AM radio test gave me a BIG clue.
Beyond that, always be sure your entire AC mains wiring is totally correct, and up to "Electric Code" requirements. "Ad-hoc"/random ground wiring and ground rods are both dangerous, and can contribute noise.
In addition, the June 1995 AES Journal has the MUST READ articles regarding this important subject. See: AES Special Publications: Journal Issues
Reprints are $10 to Members (like me <g>), or $15 for non-members. Money well spent, IMHO.