“The Noise” - Suspicious sound.
My M3, which I bought in Belgium, had some faults. Some of these flaws I knew when I bought it, but others I only discovered later. That's the way it is - and if you cannot live with it, then you should not buy a 16-year-old BMW M3.
One of the problems I discovered later is a suspicious sound (“The Noise”) from the engine (or engine compartment), which may sound like ignition knocking/pinging. In the cabin, the sound can primarily be heard at 3,000 rpm plus / minus and light throttling. At cruise at this speed I cannot hear the sound and at full throttle neither.
It can be something completely different - for example a component, heat shield or the like, which vibrates and then it is a trifle. But if it's the ignition knocking, then it's serious and MUST be corrected.
I guess the reason I did not discover this sound in Belgium was that it was raining the day I test drove and bought the car. The rain against the body must have drowned out the crisp sound.
In June (2020) I got me an appointment at Bayern Auto Group in Aarhus. It's a long way to drive, but from the past I knew André, who works here - and he knows what he's talking about in terms of M3 - not many people do that, even though they claim that "they know it all". André test drove the car and tested it through from A to Z. No faults at all on the engine, sensors, Vanos, etc. André pointed out the bearing play that it has in the differential's right output shaft and that the front rubber bushings for the triangles were probably a bit old and soft - but otherwise nothing else on the car. He could not find the cause of the "ignition knocking sound", as it also did not come from the plastic cover at the top of the engine (he found a BMW note that it can rattle as knocking if the distance to the cylinder head is not min. 2 mm).
Of course, I was sorry to have taken the whole trip from Rønnede to Aarhus with accommodation without solving the problem - but I cannot blame André or BAG because the phenomenon is obviously quite rare - and chasing sounds/noises is not easy.
Subsequently, I trawled the internet and found a BMW Service Bulletin (SI B13 01 07), which mentions something that might look like the problem. However, it turned out that this Service Bulletin is not known in Denmark (Europe), but only the USA and Australia - and that the treatment it prescribes requires the use of a special agent (Fuel Injector and Induction Cleaner Concentrate), which I acquired, but also an application device, which apparently only BAG has here in Denmark (which I found out later). A source stated that Chevron's Techron was similar to BMW's special concentrate and that it could be poured into the tank. Long story in short: I bought some Tecron and have run two full tanks with it. It did not help.
After a few attempts with my action camera in the engine compartment while driving (can only record sound due to lack of light) then I have come to doubt whether it is the ignition knocking at all.
The next step was to get a small microphone (connected to the mobile) and record the sound in different places in the engine compartment - in the hope that I could identify where the sound comes from.
I made 4 recordings, each with a different microphone location. No audiable variations were found.
A friend helped me getting and using some sound processing software (Audials), which in combination with a dash cam, a small microphone and not to forget Open Shot Video Editor enabled me to make the below video.
The video has explaining text and should be encapsulating the problem.
Video has in the beginning mostly recordings without "The Noise". This is to give a reference an an idea of, how specific the conditions for "The Noise" are. In the video RTS stands for Real Time Speed (of sounds).