...chances are...a can of contact cleaner will work wonders...

Turn off and unplug the unit...spray as per mfrs. recs and switch those switches and twiddle those knobs repeatedly to work at cleaning and reducing the oxidization on the contact surfaces...99% of noise problems (and even channel difficulties such as you describe) are simply a result of a build-up of electronic crud over the years...

jimHJJ(...yes, crud...)