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Discussion Starter #1 (Edited)
On my naturally aspirated M54, I'm getting a consistent metallic 'rattling' sound between 5000-5500 rpm on full throttle. It disappears above that. I have an Innovate LC-1 and my lambda is 0.87-0.89 during the time the noise appears, and goes steadily down to 0.85 (12.5 AFR) on red line. I'm assuming these are correct factory tuning specs (or am I wrong?). It also happens regardless whether I use 89 or 93 AKI octane.

There are no error codes revealed using a scanner. My LTFTs at both banks are around -8.0%. No vacuum leaks, healthy fuel pump, 1 yr old fuel filter.

It sounds like a mixture of 'electronic static' and thin metallic sheets rattling, and the noise doesn't change frequency through that rpm range.

Is this pinging? What does detonation/pinging sound like in an M54? Or is it some loose metal sheet probably an exhaust heat shield that just happens to resonate only during within that rpm range?
 

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First off, 'Detonation' is a misnomer, and doesn't exist

Secondly, that sounds like it could be pinging, it could also be valve train noise. Try putting some octane booster in your fuel and see if that helps. If not, have a mechanic have a look.
 

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Discussion Starter #4 (Edited)
I doubt it's valve train noise since the sound doesn't change in pitch or frequency on rpm. I think it's detonation/pinging/knocking (choose your term) but I'm hoping it's not. I'll put in the octane booster and see if the noise disappears.

I'll check the DISA too since it's easy to remove anyway.

Does anyone know the standard factory AFR range of the stock M54 on full throttle? Knowing that info would help diagnosing if this really is lean-mixture induced knocking. Also, are the knock sensors pretty much useless then if this was indeed knocking?
 

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Discussion Starter #5
DISA it is!

Just an update ...

I removed the DISA valve and found out that the flap had some play in its shaft in the default open position. It was freely rotating about 5° on both directions without tension. I checked a friend's DISA unit and his was firm. Changed the unit to a new one (11-61-7-544-806 supercedes 11-61-7-502-269 for the M54B25) and everything's well again.

At first I was skeptical the noise was from the DISA, since most DISA issues I've seen happens in the lower rpm range. Apparently the loose flap resonates with the intake air rushing between the runners during the 5,000-5,500 rpm range where I heard the rattling.

The DISA closes below ~4,100 rpm to simulate long runners, while it opens above that to simulate short runners. BMW did a good job in designing the intake, but as usual, didn't do good in choosing stronger parts/materials. Bosch is the manufacturer of the DISA. Hope the updated part lasts forever.

Thanks to TxZHP04 ... again for making the correct diagnosis. :bow:
 
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