If you substitute another speaker for the center, is it still there? If it is on another speaker wired in the center channel, this would eliminate the speaker itself. Have you verified the connections of all interconnects and speakers as being correct for phase, and if they have metal casings, they are not shorting on the equipment cases?
Does it happen on low level (volume turned down) listening/watching of any other type? If so, you may have a ground loop problem....but it should be audible on all recordings.
I would hesitate to say based on what you have provided that it is the amp/receiver causing the buzz.
Regards,
BBIBH
