Just bought a fender rumble 75 nice amp sounds good don't play shows so it's pretty loud for my garage. But when I power it down there is a high pitched hiss or pop right when the unit shuts off is this normal or ok?
SS amps can make all sorts of extraneous noises on power up and power down. If the amp is working properly I don't think you should worry too much.
My acoustic head also does a bit of a hiss after I turn it off - I think it might just be the capacitors running out of charge. Whatever it is, it's perfectly normal as far as I know.
Yes its normal for some amps, you'll probably find that turning the volume all the way down before turning it off will eliminate the noise.