Select an input that is not connected, for example select CD with no cables going to the CD input on the backside. Will you then hear equal humming on both channels? If so, the amp stage is ok,
and the problem lies in the signal level circuitry.
Open the cover and use a plastic (insulated) stick to touch suspectible cables and components. When the sound returns, you've found the cause. Resolder or reseat the component/cable/card.
Use the insulated stick, NOT your fingers!