When turned on amp it made loud humming noise. Checked cables, turned off and on, changed outlets, and still had the humming. I slapped the head and it quit the humming and started working fine. Turned off and on several times over the next few hours with no problems. The following day I turned it on and had the same humming again. I hit the head again and the humming stopped. Have been turning on and off over the past few hours and seems to be fine. What could be causing this? Anyone have any ideas? Thanks.