The fan. Any good tech can get in there to clean the unit and if you are very lucky the cleanup will help. Fans do not last forever so get a quote from the shops on how much to change that fan.
Our Panasonic Receiver, which also provides us with DVD, AM/FM, and AUX for Sound Surround has been acting weird. We've had it about 5 years now. While watching TV using the AUX function enabling the sound surround, the unit flips over to what sounds like DVD disc function without any input from us. Also there is this weird "Whirring" noise that is constant. If the receiver is bad are my 4 component speakers no longer usable?