actually… I just remembered that there is a bit of ground noise hum with my Sequential Pro One. I have to use a three-prong adapter with it. otherwise this noise is pretty bad. have you tried something like that? are you sure it’s not just a ground noise? (if you’re outside of North America, I have no idea what the equivalent of something like this is. if there is one.)

this doesn’t necessarily mean there’s an issue with the synth. my Pro One has seen a tech while it had this issue. he checked the synth out and said it wasn’t.