your purity rings may either be out of adjustment (or the tabs were broken off) or the yoke may have shifted backwards. the purity rings are the set of 2 rings closest to the tube; with a sharpie mark a line across the rings front to back so you know where they were adjusted originally. you can try spinning the tabs until it clears up. if you're unable to get it to adjust out then loosen the set screw on the yoke or try turning it as is first. if it turns, then the set screw is too loose and it could have potentially slid backwards. loosen it up and push it as far up to the tube as you can, then tighten. if the yoke moved, you can try the purity ring thing again too, it should eventually adjust out between the two.
if that doesn't clear it up then you might have magnetic interference from the speakers or something else.