[Mondial]

The only thing I can think of is some sort of mechanical interference with a wire or other component at that certain position of the dial. Is there any wire or other conductive part close enough to touch the dial or gears of the tuning mechanism?

The part thats confusing is the fact that it is not volume dependent. This pretty much eliminates the tuning cap itself since any shorting of the plates would cause a noise in the RF and IF sections which would vary with volume setting. I think the tuning cap is isolated with rubber grommets, so possibly there a bad ground to the cap frame when tuned to the problem region of rotation.

I would look for a mechanical cause somehow related to the tuning cap mechanism itself.