The preamp or source can be the problem. Run your tuner or something else for the same time or longer. If it still does it, then maybe it's the preamp. There may even be a new transmitter in your area that is getting in your system.
The RF (radio frequency) can be getting into the system somewhere. The carrier wouldn't be in the hearing range but still getting amplified, and your speakers may be trying to reproduce this RF, using a lot of power trying. Sometimes getting rid of it can be a problem. It can leak in through the interconnects, or somewhere else. I'm not saying this is the cause, just a possible one. Better shielded interconnects may help in some cases.
Not too long ago, I went past the water tower for our area. It has so many antennae on it, the tower was almost hard to see. Years ago, a small AM radio antenna was put up about two or three miles from my home. This came through our phones, and played music for the people we put on hold. For some reason, one line was louder the other. The music signal (not the RF carrier) was in the audible range. If it was digital, I may have had no idea it was there. I still have all kinds of home-brew RF traps in the wall jacks to block it, if that RF is still there.