The reason why I tended to think that it was something other than a bad bridge ground is that it happens during palm muting, when the palm would be touching the strings. I had a lot of feedback problems with pickup vibration on one of my guitars, so that's the first place I'd look, but by all means ensure that he grounding is all working. You can use a continuity checker function on a multimeter (not all multimeters have this, check yours) to test the ground.