Hi, I bought an old GK 115T to use with a 400RB. It all sounds great until I hit certain notes on the E string (especially the 12th, 13th, 1st fret and open E) where the speaker rattles/farts pretty loud. The speaker looks ok from the outside. Anyone have any ideas what it could be and how to solve it? Also, how to open the cab? There aren't any screws anywhere. Here some pics. Thanks!