Power off the receiver and all components connected to it.
Wait 2 minutes.
Unplug the speaker cables from the receiver.
Check that both speaker cables are properly installed on the speakers.
If necessary, plug the speaker cables back in.
Then power on your receiver.
Wait 2 minutes.
Then try playing music.
At this point, you’ve power cycled the receiver and any residual electricity that has built up between the receiver and speakers.
If the problem still remains, One or both of the internal solder joints/ends of the cables inside the receiver for plus and (or) minus on the defective side is facing a loose connection or corrosion. This is an easy fix..just have a repair shop unsolder and resolder/clean the bad end(s).
I can’t say 100% that this will even help you, but in my experience working with a lot of amplifiers over the years, I’ve had the same issue numerous times (lots of 1980s gear as well)