Engine metallic rattle
 

X5 makes a metallic rattling sound only when the engine is cold. As soon as it warms up the sound is gone (or atleast I can no longer detect it). The sound is coming from the engine compartment and is RPM sensitive; i.e., sound gets faster as engine is revved. It's almost like a light metallic ringing sound. Can't really explain it better than that - any ideas?

Does it make the sound for a few seconds or minutes when you start it?

I'd like to know that also.. Mine does that for a sec when cold. I Think its the chaintensioners in my case.

Engine rattle sound
 

It does it mainly when it's cold upon start up. Sometimes, I think a few seconds, but not always.

Cam chain tensioners or possibly hydraulic tappets (bit more of a clatter)

If you have the m62tu engine with the original chain tensioner then you can try switching it out with the updated version - Home Page > Search > 11317531813 > ES#22304 Timing Chain Tensioner - 11317531813

It's not too hard to replace just a little hard to reach and if you still have the rattle after the switch, then I'm sorry to say your chain guides have broken up and need to be replaced.

Engine Rattle Sound
 

The engine rattle sound turned out to be a faulty alternator.