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It's not a belt but ...

I think this might be very minor timing chain slap caused by insufficient oil pressure to the cam chain tensioners(s) or possibly 1 or both tensioners themselves.

The only other possibility I can think of is possible a vanos problem.

Check your oil pressure against specs. with a true gauge. Also, this could be a symptom of using an oil with low viscosity.

Lately I have been tempted to try one of the vanos seal kits offered just because it's about time for mine to need it. You could try this if your mileage is getting up there.
 

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I've never been clear on the "marbles in a can" rattle the VANOS is suppose to make, could be that though.
 

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If it only happens when it's cold outside and it goes away after about a minute, could be secondary air pump, the mushroom shaped thing on right side above exhaust manifolds. Put your hand on it and feel if it's vibrating (pumping air into head) while you hear the noise. Doesn't sound like VANOS rattle or lifter tick to me. Could be you're low on oil (check dipstick) or your timing chain tensioner has failed. Mayyyybe it could be a failed belt tensioner or idler pulley, but I suspect those usually make more of a squeal or screech when they're failing.
 

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If it only happens when it's cold outside and it goes away after about a minute, could be secondary air pump, the mushroom shaped thing on right side above exhaust manifolds. Put your hand on it and feel if it's vibrating (pumping air into head) while you hear the noise. Doesn't sound like VANOS rattle or lifter tick to me. Could be you're low on oil (check dipstick) or your timing chain tensioner has failed. Mayyyybe it could be a failed belt tensioner or idler pulley, but I suspect those usually make more of a squeal or screech when they're failing.
Truth. That noise doesn't sound like my failed bearings.

Note: Noises mainly in cold weather w/ rough idle in said cold weather is considered indicative of a VANOS problem...(just throwing that in, food for thought).
 
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