Ground loop or more likely failed component.
Check you have a proper ground (no cheater plugs)and try plugging everything into the same recepticle (OPPO 983 and Amp) - use a power bar if necessary. If that does not cure it then one of the power supply caps or a transistor may be on its last legs (works cool but fails under stress/heat). (Some type of caps will arc as they fail and may self repair but if it is dried out or seriously damaged then you'll need to raplace it)
Another way to avoid/minimize ground loops is to use TOSLINK from source to amp or XLR rather than RCA type analog connections...
Check you have a proper ground (no cheater plugs)and try plugging everything into the same recepticle (OPPO 983 and Amp) - use a power bar if necessary. If that does not cure it then one of the power supply caps or a transistor may be on its last legs (works cool but fails under stress/heat). (Some type of caps will arc as they fail and may self repair but if it is dried out or seriously damaged then you'll need to raplace it)
Another way to avoid/minimize ground loops is to use TOSLINK from source to amp or XLR rather than RCA type analog connections...