Are your doors making a creaking sound when you hit a bump, like they are slightly moving and it doesn't seem to come from the interior door panels? Then check for the protection tape on the chrome above the doors. We recently discussed that on the German E32 forum and there is a retired former engineer from BMW which was working in the production line for the E32 in those days. His comment:
Das Band war Serie. Aber nicht ganz vom Start weg, es müßte so um 89 eingesetzt haben. Bei Frost konnte die kaschierte Tür Dichtlippe an der Zierleiste festfrieren... und abreissen. Das Band hat das verhindert. Es gibt auch noch den zweiten Grund: die Dichtlippe am Fensterrahmen ist kaschiert, beflockt. Auf dem nackten Alu reibt sich diese Beflockung relativ schnell ab und es kommt dann zu Quietschgeräuschen Gummi-Alu. Das Band verhindert den Abrieb.Der dritte Grund ist rein optischer Natur. Stimmt die Einstellung der Türen nicht und es gibt einen Höhenversatz... oder die Türdichtungen schrumpfen mit Versatz... dann sichert das schwarze Band einen optischen Fortgang ohne Stufe. Es schönt die Optik an der B-Säule.

translated:

The seal/tape was a standard. But not quite from the start, it should have be introduced into the production line around 1989. During cold temperatures/ frost the laminated door sealing lip could be frozen on the trim strip ... and torn off. The tape has prevented this. There is also the second reason: the sealing lip on the window frame is laminated, flocked. On the naked aluminum this flock rubs/wears relatively quickly and then it comes to squeaking noises rubber <> aluminum. The tape prevents abrasion. The third reason is purely optical. If the adjustment of the doors is not right and there is a height offset ... or the door seals shrink with offset ... then the black tape ensures an optical progress without steps. It improves the looks on the B-pillar. Here pics of the tape , see large pic in post 15 http://www.7-forum.com/forum/5/klebe...te-227871.html
http://www.7-forum.com/forum/5/rein-...em-217364.html
that tape is not available from BMW but comes with the complete chrome trim. In Germany they have solved this by purchasing on the internet chrome decor trim tape matt black 10mm like this
http://www.ebay.de/itm/221354396099?...AIT&rmvSB=true
temperature resistant -40°C to +125°C , Stärke: 0,95 mm