Strange Engine noise

The car is a 330i E46 ...latley I have had a strange rhythmic thumping noise at low RPMS the lower the RPMS the worse it is ...Here is a link to a video of it ....you can really hear it well near the end of the video..Any Ideas would be greatly appreciated..

http://www.youtube.com/watch?v=BnYO2tY8y8A

Well as usual with most posters, you left out MUCH INFORMATION to make an educated guess. How many miles are on the car? Have you had any services lately by the dealer? The dealers catch many things as their mechanics are trained to check many things you would never think to look at.

Now with all that being said, it almost sounds as if the serpentine belt tensioner or something on that belt (alternator bearing, water pump bearing etc.) is on its way south. At first I thought it could be the camshaft drive chains rattling around but it's not metallic enough. Something does sound like a bearing is going out. Does the noise change in conjunction with increased RPMs? Just run it at idle then run it 1000 RPM faster to see if the noise pulsates along with the faster engine speed.

If you're not a mechanic, I suggest you go see one ASAP so you don't get stranded when what ever it is breaks! It could be something as simple as re-tighting the belt tensioners if it has come loose. It could also be the spring in the tensioner itself went bad.

Good luck & Merry Christmas!