Just bought a used Flextone III.  Tonight there is a noticeable buzz..almost like a ghost of a distortion patch coming through whenever a note is played. It is not mechanical, as in speaker buzz/ screw loose etc.  It is there as the clean note fades out..if you play 2 or more notes together, the distortion comes through more audibly...really destroying the sound of the clean tone after the initial note. It doesnt matter the level of the gain/volume/master knob.

Any idea? Cant seem to find anything about the same issue.