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Old 07-29-2021, 03:11 PM
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bcredliner bcredliner is offline
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First, independent has said what they think isn't the problem, have they done any troubleshooting or said what the problem might be?

If there has been a check for trouble codes that is the first thing I would do. If it is the transmission related there should be trouble codes. There should be an error code for a vacuum leak and engine wouldn't run as it should.

Sometimes the circumstances associated with a problem are misleading and should be set aside, although, the sound usually helps. I agree that it sounds like a bad bearing or the clutch fan but that is a guess. When troubleshooting I don't remove anything unless it is absolutely necessary. I have damaged a perfectly good part, damaged something else in the process, broken a bolt off, made a mistake in reassembly and had to take it apart again etc. After checking for codes, I would start troubleshooting with stethoscope on the intake and then trace the sound to where it is the loudest. I don't think the engine will need to be hot or the sound present. If there is a problem with a bearing you should hear kind of a grinding noise rather than a constant hum. If you don't hear it cold try when engine is at operating temp. If you loosen a tensioner you should be able to tell if bearing is wobbling or bad without completely removing the belt. Check to see if one of the brackets are loose or broken at the same time. I never run the engine without all belts in place. It's too easy to overheat the engine and the block being aluminum that's high risk. Im addition BMW electronics are very sensitive to low voltage.

Secondary air pump comes on with a cold start to warm up cats. Usually turns off around a minute. It's unlikely it comes on after 30 minutes or stops making noise per your circumstances.

If you have always had the AC on when the noise starts turn if off when it happens so the clutch disengages.

I agree it could be the clutch fan since the level of engagement depends on the amount of heat reaching it. It should spin easily when engine is cold and be running consistent with RPMs when engine is at operation temperature.

There are other potential causes. Troubleshoot, troubleshoot, troubleshoot. It's not yet time to buy any parts.
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Last edited by bcredliner; 07-29-2021 at 03:21 PM.
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