[jerryhawthorne]

Eric, your on course! The little tapping indicates either a very microphonic tube or perhaps a bad connection. If the only place you find it is on the FM 100 tube, clean the tube pins and the socket with something like Deoxit or even WD 40. A little of that on the base of the tube with some fine steel wool. Clean off the base well with some clean rag to assure no steel wool remains and then insert multiple times in the socket. Could be just a bad connection between the two. Of course it could be the tube, but I doubt it.
Jerry