Sounds like a capacitor failing in the power supply. It may be arcing at its own frequency determined by the progress of its failure. It would be one of the types of failure that could affect both channels. I recommend taking it in to an authorized Sony electronics repair shop. Good luck!
Need Help
My dad has an older Sony reciever.There is a LOUD buzz in both channels.The only source hooked to it is a CD changer.I had a ground loop hum in my system a while ago.This is not the same noise.I checked all the connections,everything is hooked up right.The outlet is two prong,so he is using a power strip with a cheater plug.I tried plugging in the reciever directly into the wall,same results.Any help would be appreciated.Thank You. John