Vintage Synth Repair

My Rolans RS-09 makes a high pitched wine like a TV (well maybe not as
high as that but similar) when not being played..it's not very loud
and it's still useable but it's very annoying. I guess it's about as
loud as  the hiss. It's sounds like a high pitched organ tone stuck
on. It's not supposed to do this right? Any info on fixing it?
schematics would be nice :o)

Thanks, Phil.