Mysterious noises M275 AMG SL65
 

Recently purchased this car knowing it had this rattling sound. Only noticeably audible from inside the cabin when warmed up and idling at low rpm. Noise goes away if I hold the throttle and raise rpm even slightly; noise goes away once I'm driving forward/backward (basically above idling RPM). Nothing changes whether I'm in neutral or reverse or drive; nothing changes whether I control the fan blower direction, temperature, or AC. I've noticed at night time (colder temperature), the noise appears a lot later (after a longer warm drive), and also sounds quieter compared to a sunny warm afternoon. If I shut off the car for 10min or so, I can startup with a chance the noise not being present, but would shortly appear again after it warms back up.

2 videos, inside and outside. Inside is very obvious. Outside not so much, even if under the car or inside engine bay.

Inside:
https://drive.google.com/file/d/1PxY...ew?usp=sharing
Outside:
https://drive.google.com/file/d/1AVb...ew?usp=sharing

I am clueless, anyone have any ideas?

Welcome to the forums.

Good videos.

I would have a look at the clamping bands that hold the exhaust to the turbos.

After that check the exhaust mounting brackets.

After that the heat shield mountings - there are lots of them.

Are there any faults reported or other symptoms observed? Do you have access to SDS or an OBD2 reader?

Nick

Okay, I'll check all those 3 things very soon. Thanks for the direction. The only fault I have right now is relating to my Secondary Air Injection. I doubt that is related to this noise though. I have an Autel MOT Pro to pull codes and read live data if needed.

I got the car on a hoist and started inspecting the sound. I used a long screwdriver against my ear as a stethoscope. I've noticed that if I listened against the underside chassis driver side, the knocking sound would be more obvious than on the passenger side.

When I listened on this particular heat shield, I can isolate the noise. Moving or flexing the heat shield did not make the sound go away or any quieter.
https://cimg9.ibsrv.net/gimg/www.mbw...fe44f24ffd.jpg
https://cimg1.ibsrv.net/gimg/www.mbw...c5e041fb23.jpg

When I listened on the heat shield that surrounds the steering shaft, it is the loudest and most obvious. I noticed that it was much more loose compared to the other heat shields, but moving it around didn't make the sound go away either.

https://cimg6.ibsrv.net/gimg/www.mbw...62e50f1abd.jpg
https://cimg5.ibsrv.net/gimg/www.mbw...d1036d18a6.jpg

Everything else I listened to, such as the cat converter, turbo, manifold, transmission, motor mounts, does not create or transfer that knocking sound. I am really confused as to why this noise would occur only when the car hot, and only at idling rpm. It seems like the noise is coming from the driver side firewall. I'm trying to look at component diagrams of the car and figure out if there are any motors, solenoids, etc, that exist at the driver side firewall, that maybe, once hot, would produce that sound. The most obvious source of the sound was when I listened to that heat shield surrounding the steering shaft.

bump. Anyone have any inputs?

Did you listen to ABC pump, a/c compressor or alternator? Try removing the engine belt and see if noise is still there

Yup, I probed those and the clicking wasn't from there. Like I mentioned, it was most prominent and obvious when I probed the chassis, front driver area. I could try and remove the accessory belt and run it, but will it be an issue on these Mercedes if I start & run the car without the alternator running?

I've recently purchased the car, not sure if they've been recently replaced prior. I did probe both engine mounts and they don't create or transmit the sound.

Does the noise change with accessory loads or if the AC is loaded/unloaded? This noise sounds like a damper in the AC pulley or the ABC pump itself.

No it doesn't. The only thing that affects whether the noise appears or not is if the rpm is low enough, idling, and if the car is warmed up. Also like I mentioned, I did probe the area near the driver side firewall, and the steering column heat shroud is the point where the knocking sound is the most clearest through the stethoscope.

since you own the car now maybe contact the PO and ask him to come clean on anything he may know concerning the noise. I would get a cheep HF stethoscope and isolate the noise. You will be able to narrow down the area. Without the scope there's just too much coming at you.

To this day, still have not figured it out. The source of the sound is somewhere in the firewall area, underneath the windshield wiper unit. At this point, I'm guessing it is something loose that only vibrates when the car is fully warmed up and is allowed to warm idle at 500-600rpm. The noise gets annoying. I'm looking at potentially raising the warm idle rpm to maybe 600-800rpm just to make the noise go away. I know this would work because as I've mentioned before, I hold throttle a tiny amount so it cold idles at ~800rpm, rattle goes away. As well, when the car is still cold and the idle rpm is above 750rpm dash line, noise is not present.

Does anyone know if a Mercedes das/star xentry (im not familiar with these OEM softwares/tools) has the ability to adjust the warm idle rpm and raise it?
If someone can confirm it's not possible to do that, then the only other method I can think of to adjust the rpm is to 'trick' the ECU by modding the coolant temperature sensor. I could raise the warm idle rpm is by adding a resistor rated somewhere between 30Ohm-150Ohm in the wiring connector to the coolant temperature sensor. I have previously worked on other cars and used this method to 'trick' the coolant temp in order to adjust the warm idle rpm this way. I'm not sure how these Mercedes' ECU works entirely, but I know most cars have a proportional relationship between coolant temps and idle rpm. If these Mercedes works in the same principle, then I should be able to trick the ECU into thinking the coolant temps are at 80°C and keep the idle rpm in the 700rpm range.

No, warm it up until the noise appears, and then remove the belt.The car can work for some time with spark power just powered from the battery. But, I would not let it run for too long, because of the water pump(s?).

br,
syljua

Yes, do this. Raising the RPM to remove the sound is not ideal, that noise shouldnt be ignored.

You can raise the idle by 50rpm in STAR.

What's the difference between ignoring the noise, and raising the idle by 50 RPM? Whatever it is may not rattle any more, but it's still not actually fixed.

It's possible you just have something that was dropped down in there during service. Don't know how many sockets and screwdriver bits I've left in cars over the years.

And... have you looked around underneath and behind the dash while the noise is occurring? There's plenty of trim and plastic bits there that can rattle around under the right conditions. Left side fuse panel cover night give you some access, then there's the little OBD connector cover, all that stuff. Just a thought, you did say the noise is more obvious inside the car.

Have I read about an ABC line that runs somewhere near drivers floorboard that can make contact with the body and make noise under certain circumstances? Maybe it was S Class and not SL. Not sure. How many miles? Accumulators/pulsation damper could be going out.

Hello there.

Did you ever found the cause of this mesterious noise?

I have small mercedes shop, and I have a customers SL65 with the same problem (noise), exactly the same.
We already change the power steering /ABC pump, and still there.

This begin since the car had a small steering rack leak. We remove, fix and replace the steering rack.
After that my customer says that the noise wasn't there before. Im not sure if that was true or not since I didn't drive it before we work on it.

I notice that if I could raise the RPM 100+ the noise would disappear at 90% of the time, and it wouldn't affect the driveability of the car.
I do have a star system but could not program it to keep it 100+ RPM at idle.
After raising it, and going out of the system, the RPMs goes back to the OE program.

After reading your post, and read others responses we will check all the heat shields that the car has. Some of them were removed when we work in the steering rack, maybe we left one loose.

Anyways, I hope you found the cause and post what it was
Cheers.

This sound is almost certainly a lifter. I had the same sound, it went away once the lifters were replaced.