A freind of mine at work had that problem in his 325. In his car, the rear glass weather stripping was failing and had gaps in quite a few areas. I'd check that out first.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>