TomK [MA][99 BW 5.9L FT4WD]
tek@acm.org
or, it could be a bearing somewhere, a "mechanic's stethoscope" would be able to pinpoint it (or, a trick I learned from my brother-in-law, you can use a long screwdrive and cpu your hand over the end and against your ear.. but be careful, there's lots of stuff moving around in there!)
...tom