I've got this periodic low-freq thump in one channel of a preamp, and I've traced the source to right around the area of a transistor. It is a bit tedious to replace this transistor so before I begin, I wanted to ask if periodic low frequency "thumping" could typically be caused by a bad transistor (or more likely another problem?)? On the scope it looks like just the top portion of a (slightly deformed) square wave going by every few seconds or so...