[ICOM] Noisy 706

[Al Waschka]

I have an original 706, not MkII or G, that has developed an intermittent noise.  It sounds like atmospherics and static crashes, and it is affected by the preamp switch.  It moves the S-meter.  It is loudest with the preamp ON and gets softer with the preamp OFF.  I can't tell if it is still there with the ATT on.  The noise is not coming in through the antenna; it is unaffected whether the antenna is connected or not.  Anybody seen anything like this?  Nothing on the ICOM knowledge base.
 
Thanks and 73,
 
Al K5TAN