I'm wondering if it just sat so long that the relay contacts might have been dirty and allowed a signal to the amp output that would normally short to ground when powered off. I am assuming this amp has such a relay and circuit, and I don't know that. It's a model I haven't used or sold. I know this isn't an old product, but it seems a lot of the older amps from the 70's and 80's made funny (or scary) sounds when powered down and connected to speakers back in those days. I may be thinking about Phase Linears' or other less than pro product. Sorry about the earlier attempt at humor.