Another weird noise - not rocks, rear end, a/c drip or cooling!

[Georgeair]

I searched for this one, but can't find a similar discussion.

I've had a strange metallic sound from the left rear seat off and on for a while now. It sounds like a thin metal on metal rattle, coming from the left rear seat area. Sort of hard to look and drive from there. I can replicate it every time, mainly present when accelerating gently away from a stop, between 1200 and 1500 rpm. Yes, I have an Eisenmann Race, but although that may be causing the resonance to trigger this I am 99% sure it's from inside.

My first thought (from a similar problem on another car) was the seatbelt buckle rattling against metal or hard plastic, but that's apparently not the case.

Wife riding with me today and stuck her head back there, thinks it may actually be coming from the speaker in the left side panel. I have Enhanced Premium Sound option, it has not been run hard.....

Any ideas or suggestions that have come from others who have already solved this one? I'm due for a service in a month or so, and will have them look at it then but can only imagine the difficulty they will have re-creating it without letting me take someone for a spin.

[C17MooseDriver]

Mine has a plastic rubbing on plastic sound coming from the left passenger seating area. I think mine is from the fold down seats though. Nothing metallic sounding however.