Occasional Rattling noise on cold startup

wormtownhero · 12-02-2020, 06:19 AM

2011 328i xDrive N51 SULEV 104k miles. Occasionally when I start the car from cold I hear what sounds like a brief second or two of ball bearings rattling around the motor and then it goes quiet.

Could it be the belt tensioner or pulleys? I purchased the car second hand at 70k two years ago and have not done this particular service. Belt and pulleys aren't new but visually look fine and I never hear the same noise again driving or subsequent warm starts which makes me think it's something to do with those pulleys or belt starting to go or perhaps something to do with the N51 engine. Thanks!

NYCiROLL · 12-02-2020, 10:48 AM

I'd recommend getting a mechanic's stethoscope. (looks like a Dr's with a hollow tube at the end) It's excellent for isolating where a noise is coming from.

Being that your car has a N51, it could be one of the disa valves in your 3 stage intake.

If the repair records don't show the tensioner/ belt being replaced it could be the bearings there or in your alternator.

My 2 cents.

12-02-2020, 06:19 AM	#1
wormtownhero Registered 0 Rep 2 Posts Drives: 2011 BMW 328i xDrive Join Date: Nov 2020 Location: Worcester, MA iTrader: (0)	2011 328i xDrive N51 SULEV 104k miles. Occasionally when I start the car from cold I hear what sounds like a brief second or two of ball bearings rattling around the motor and then it goes quiet. Could it be the belt tensioner or pulleys? I purchased the car second hand at 70k two years ago and have not done this particular service. Belt and pulleys aren't new but visually look fine and I never hear the same noise again driving or subsequent warm starts which makes me think it's something to do with those pulleys or belt starting to go or perhaps something to do with the N51 engine. Thanks!
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12-02-2020, 10:48 AM	#2
NYCiROLL Enlisted Member 37 Rep 43 Posts Drives: MSport E91 Join Date: Oct 2014 Location: NYC iTrader: (0)	I'd recommend getting a mechanic's stethoscope. (looks like a Dr's with a hollow tube at the end) It's excellent for isolating where a noise is coming from. Being that your car has a N51, it could be one of the disa valves in your 3 stage intake. If the repair records don't show the tensioner/ belt being replaced it could be the bearings there or in your alternator. My 2 cents.
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