My 02 745i just started doing exactly the same thing a week ago. On a cold start I get a grinding noise whose tone goes up and down with revs. After about 60 seconds, its as if oil has reached the non-lubed part and then goes quiet. My fear is that it sounds like cam bearings cos the sound is definitely coming from the driver's side bank. Cannot explain it but hope my BMW can resolve. After a minute the noise is completely gone and does not appear again until the next cold start. Anybody had similiar issues. Sounds like a dry tensioner pulley or something like that.