| OK now you have given some information that can help...
the amp cycles on/off during a turn! Did it change as you were jiggling the wires on the outside of the amp? a wire (speaker, power, RCA, or the turn on wire could be loose or pinched) again the wiggly jiggly should tell which it is. DONT FORGET the sub, if the wire connected at the woofer (in the box) is loose or was pinched the sub could be making partial contact and that could sound like the amp is cutting on and off. If the AMP is cycling on and off there is probably a momentary cut off period (one or two seconds) and then the amp will fire back up
The amp may not be cycling on and off you may be loosing signal from the RCA or the speaker connections.
ALL the above are free fixes (that is these are the failures you want because they generally are not dangerous and they are free to fix). Once you have completed this testing (be thorough) then you are left with a fault in the amps internals (you might recall doing something violent to the RCA input or to a speaker output).
I dont know what could be making a hissing sound? | 
