Hmmm. Sounds more and more like a bum control. Any sign that somebody bumped his knee on that knob, like a bent shaft, maybe?
Don't have a handy pic, but the quick test for this is to turn the control full to the right, and use the tip of a small screwdriver to bridge the center lug of the pot to the clockwise lug.
If the control has gone bad, this will get you your noise level back until the screwdriver tip is removed. It can be important to turn the knob full clockwise before doing this. Bridging the wrong end-lug of the control to center can damage the control in some radios. Probably not this one, but it's never a bad idea to use care.
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