Re: Randall RH150 G3 effects loop
almost sounds like a grounding issue. did you double check to make sure you are using instrument cables in the FX loop, and not speaker cables ?
It is also possible the FX loop jack has lost it's ground, or has a cold solder joint.
Have you tried non distortion/boosting pedals like a chorus or something like that ?
does it have send/receive level controls?
I saw a similar post over on sevenstring.org . They got a whistle just plugging straight in to the amp though. Could be a similar issue.