Originally Posted by M3PO
I personally believe that it sounds like timing chain slap. There are valves on the hydraulic timing chain tensioners that hold in oil pressure to keep the chain tight while the engine is off. If the valves on the tensioners go bad, the oil pressure will bleed off allowing the chain to become a little loose. When the engine first starts and oil pressure is low, the chain will rattle until oil pressure builds up and reapplies pressure to the tensioners and chain.
BMW seems to agree. They are changing my timing chain tensioners as we speak. However, another member posted that this didn't solve his problem. In one case, replacing the VANOS check valves seemed to do the trick but that didn't work in another. From what I understand, there are pins on the VANOS that lock the system in place when the engine is off in which case the check valves don't come into play at start up. Again, my dealer/BMW seems to agree with this. They disabled the VANOS system (disconnected the VANOS solenoid IIRC?) and the problem was still present.
edit: I should also mention that nothing suggests that this is a bearing issue.
It is a weird one. Before the engine was replaced in my car by BMW, they tried:
- VANOS check valve replacement
- All tensioners and belts on the front of the engine
- Cam chain and tensioners
- Bottom end bearings and crankshaft
- Front exhaust section
None worked. That was my car in particular of course - so it doesn't mean some of these ideas won't fix another.
Each time the foreman went through their next course of action with me, it sounded completely right and feasible - but never worked.
I hope they learnt was the issue was from taking apart my old engine and have formulated a plan to rid the noise in other cars in the future (should it show)