It could be the fireplace damper that's rattling. I had an old one that never open/closed securely. It looked like this...
http://www.aaa-inspect.com/M%20Fireplace%20Damper.JPG
I'm going to mod my CJ PV12 with Teflon caps soon. That preamp clicks so nicely with my Thiel CS3.6. CJ charges $800 for the upgrade and http://www.soniccraft.com/ said they'd do it for $560. I always regretted having my subaru worked on by non-subi techs, so I'll probably regret this as well.