first of all, unplug the input from the receiver and verify that it is the receiver, and not the external sound source (TV or CD) that is producing the noise.
if you verify that it is only the receiver,
Mainly, a power supply capacitor that breaks down after it's been on for 5 minutes, or a transistor that is defective and slowly overheating.
anyway, you need to take it to the service techs service center. you can't fix that yourself.