I have an Onkyo tx-sr606 receiver. 5 to 10 minutes after turning the unit on, it starts to buzz and get louder with time. It does this with or without components or speakers attached.
This kind of noise can be caused by a transformer that was poorly made.
Iron core transformers are normally 'potted' in a vacuum so that epoxy (or lacquer) is forced into the laminations to prevent them from trying to vibrate with the line frequency.
If you are adventurous, remove the housing, turn on the power and using an insulating probe such as a plastic rod (a plastic picnic knife will work), place the tip of it on any larger parts you see and place the other end behind your ear.
This will carry the sound in a more directional manner to your ear.
You should be able to locate the source of the buzz.
If you can, and it turns out to be a transformer, it will probably have a metal shield around it so it might not be possible to gain access to the laminations.
If the shield isn't ultra tight, you might be able to flow some 'Super Glue' into the cracks and with luck, 'glue' the laminations together.
Super Glue (or equivalent) is a good insulator so will not cause any electrical problems but take care not to get it on any plastics that are exposed with the housing on, it will cloud them forever.
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