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      11-26-2021, 08:41 AM   #18
Zircular
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Drives: N/A
Join Date: Aug 2021
Location: TN, USA

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I figured as much from the sound. Nice that BMW is going to work with you on troubleshooting, although I think you'll be proven right on the outcome. Very few "my car whines" complaints on these forums seem to be resolved with a repair. (I've probably been through every thread at this point...quite the rabbit hole.) Yours is very likely the "ZF whine." It's pretty distinctive, seems to be a common annoyance, and doesn't seem to have any mechanical significance. People have had those boxes replaced under warranty only for the new one to do the exact same thing. I don't think I've come across a satisfactory explanation for the sound. Plus side, they never seem to blow up?

Here's my master list of explanations for generic whine complaints:
  • Belt-driven accessories (duh)
  • Timing chain problems (N20/26 mainly, others as the plastic guides age and fail)
  • Intake/turbo/waste gate sounds
  • Exhaust leaks
  • Differential gear noises (especially ///M cars)
  • “Normal” ZF 8HP transmission sounds (mainly when hot)
  • Other abnormal transmission sounds
  • X-drive transfer case issues
  • ASD acting up
  • Your significant other (typically from passenger side)
  • and “Don’t worry, they just do that”

I think these are often conflated on the forums since different sources can often be described similarly. We need a fault tree or something.

I'm still fairly convinced that my whine is coming from the input side of the gearbox, although I'm back off the ledge. I'll troubleshoot it as far as I can and then let it go. I replaced the fluid yesterday and pulled a sample for used oil analysis. If there's really a problem it should show up on the report.

I did come across one interesting wrinkle, though. The RealOEM parts catalog for manual M240s lists a "Noise insulation, Transmission Housing" part # 23108609376. What's weird is that it doesn't apply to a very small gap in production. It's listed as discontinued from 12/2016 and then comes back for the MY18 LCI (mid-2017 and on). I have a very late (5/2017) pre-LCI build, and I confirmed while I was under there yesterday that the gearbox is indeed naked. It makes me wonder if I'm hearing 100% normal manual transmission gear whine that others don't report because most cars are insulated against it. Just a theory but it seems plausible. If valid, I can claim that it's a "feature."
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