Do you live near an AM station? Does your receiver/prepro have an AM tuner?
If theres an AM tuner in your rig it could be crosstalk.
Or shoddy speaker cables.
Its probably the wires going to your rears, AM waves are rather long and AM antennas
need to be as long to pick them up, sometimes the windings on a power transformer will,
under extreme circumstance, act like a AM amtenna.
Check out a company called c crane , they specialize in AM radio and long distance AM
radio reception ( DXING)
And they might have something to do the opposite.
Anyway try moving things around, you "tune" an AM radio by moving it, a AM radio
antenna is diectional, so relocating the wires or some componets might help