So.. on my 2005 320i I had this horrible rattle/noise upon cold starts when selecting D or R. It would last only a minute or so then disappear as soon as the engine heated up somewhat.
It turned out to be the alternator pulley (a clutch pulley). Mine was seized solid. I replaced it cheaply with an aftermarket one (as BMW doesn't sell them separately) and problem solved. ....BTW its done without removing the alternator and only takes about 10min
The engine is still one of the noisiest petrol engines while idling that I have come across, but apparently this is normal
However.... a new problem just raised its ugly head... see my next post named "Aircon woes"