Hi there. I've not troubleshooted it yet but wanted some suggestions. I don't have a loud hum as such, but I recently noticed that after turning off the 990/7700 combo with XLR cables that there is a high pitched whistle after turning everything off that fades out, kinda like something is 'draining' out so it goes from high to low and is gone in a second or so.

What could cause that?