I picked up a used, good-condition db750 for a good price about a month ago. It sounds great, but I'm hearing a little faint internal rattling sound (yes I stack it on top of my cabs). When I very gently tilted the head side to side, I could hear it too. I haven't opened it up yet, and I think I'll take it to my local repair guy for that, but any ideas what the problem could be? What should I be looking for, a loose wire? A loose preamp tube?