I've noticed on my new PRRI there is a weird buzz when I play very bassy pedals (EQD Bellows, Big Muff Pi, ETC) where I hear this sort of buzzing sound if I hold the note on certain frets. I notice it most on the 7th fret E, gets worse as I turn up the volume. It definitely sounds like something is vibrating in the amp. I've tried different rooms, putting it on something soft, etc. I've also tightened all 8 bolts inside the cab, the 4 nuts that hold on the speaker as well as the 4 bolts that support the chassis. They are all snug. For good measure I checked that all the tubes were seated and nothing else was loose. Seems like some sort of perfect pitch sets it off, it does happen on a few other frets too. Any ideas?