I have a Rivera era Fender Twin that recently started to add some high pitched screech, especially when playing on the b and e strings above the 12th fret. At first I though it was the speakers but after connecting to an external cab, it looks like its not. The speakers in the Twin are real tranparent (JBLs), and this seems to reveal the screech more than the other speakers I tried, so I am still scratching my head, wondering if it may also be them. The noise is a recent phenomenon that doesn't always reveal itself but when it does the sound is a shrill overtone on top of the original note. Any ideas on what causing this or how to diagnose it further?