I recently picked up a Portastudio 424 and have discovered a problem with tape playback and recording. At random points during playback the unit itself will begin to whine very loudly from the cassette deck. it sounds almost like a squeal from a bad belt. Whenever this noise happens, there is also white noise on the outputs which drowns out most of the playback. Could it be as simple as a bad belt? How would that create noise on the audio signal? Any help would be greatly appreciated, thanks!