Just to pile onto a very old thread, in case this helps anyone else like me, who's reading to learn... My '88 328 too, had that loud, annoying passenger's right ear resonance. In my case, it was not the A/C hoses, or any of the other great suggestions mentioned here. Turned out, it was the passenger's side rear window louver (p/n 61818600), which was missing the rear grommet (p/n 61989600), allowing it to rattle in a very narrow RPM range. This grommet supposedly only belongs on cars up to 76681, but my car (78698) had a hole for it, so I sourced one, and VELATO! Problem solved! Of course, while I was at it, I replaced intermediate grommets on both sides, too, just to get fresh rubber in there, which is nice and spongy. Now I can hear only the power unit symphony, and the rush of the wind.... Andiamo!
