E46 M3 Mystery Rattle

[saxon]

Hi everyone. Brand new member here who just got his first E46 M3 a couple of months ago. It’s a 2004 M3 coupe with manual transmission and it currently has 198,000 km / 123,000 miles. I need your help. There is a bit of a mystery rattle at cold start that several BMW specialists haven’t been able to diagnose... It does go away almost completely when the car warms up, but not 100%, as you can still hear it a tiny bit.

Some important service history for context (done by previous owner and confirmed by service records):

- Vanos done in 2014, so it’s been a while
- valves adjusted 11,000 km / 6800 miles ago in Oct. 2022
- engine overhauled in July 2023 (10,000 km / 6200 miles ago) including the following:
* rod bearings
* head gasket
* timing chain tensioner/guides
* crankshaft oil seals
* drive belt idler and tensioner pulleys
* serpentine tensioner
* water pump & thermostat
- alternator replaced this summer with a rebuilt unit (one of the mechanics suspected alternator or tensioner pulley to be the cause, but looks like he was wrong)

Here is a video of the cold start. You can hear the “tk tk tk” sounding rattle begin a few seconds after I start the car.

https://youtube.com/shorts/Xb2zKxEGaxg?si=D3FNsiXplgsxqEMc

Did anyone have a similar issue? Any ideas on the likely culprit would be highly appreciated as I’m running out of BMW mechanics overhere that could give me a clear answer. Nobody knows these cars better than the owners.

Thanks in advance!

[Slideways]

To rule out something simple, did they do a cold start with both belts removed to see if the noise is still present?

[saxon]

To rule out something simple, did they do a cold start with both belts removed to see if the noise is still present?

I don’t think they did that.