Hi all,
I recently had my 2009 build in for some work. I had the front rotors, pads, and wear sensors all replaced. This went without issue. At the same time, I had my mechanic complete a visual check of the head bolts to determine if they had been replaced or not as I don't know the history of the car before it was imported into New Zealand from Japan and I'm looking at selling it. As expected the bolts haven't been replaced. According to my mechanic, they removed the left side cam cover, checked the bolts and reassembled. After collecting the car, approximately halfway home (~10km) stopped at an intersection I noticed a new 'knock' that the car was previously not making. Obviously I took the car back. They've spent a couple of hours on it, including repeating removing/replacing the cam cover, but can not find the source of the sound and have no idea how to remedy my complaint.

I don't believe the sound itself is the engine, as it isn't present 100% of the time the engine is running, but something that runs when needed. The sound isn't easily heard under the bonnet, if at all, but is very noticeable from within the car. I've attached a couple of videos. As you may notice, the speed varies and generally doesn't start for the first 3-4 mins that the car is running. From then on it is mostly present but at times, when stopping at an intersection the sound will stop ~5 seconds after coming to a stop. I've looked at the belt at the front, that seems fine/sounds normal, as does the radiator fan. It definitely seems to be coming from the side they have opened and definitely wasn't there before they worked on it. Is there something they could've knocked/dislodged which is now rubbing somewhere etc?

Any advice greatly appreciated, sorry the audio in the videos isn't the best but hopefully you can hear it. If you can recommend anything for me or my mechanic to check or look into I would appreciate it greatly.

 

cant really hear much from the videos but i'm guessing its your engine mount. there is nothing else that can produce a knocking sound if its a dense thud. maybe a the belt tensioner if that thing is real bad. any engine bay sound clips or videos?

 

At the end of my tether so paid the Mercedes dealership in the city to figure it out. Within 90 minutes a call to say the MOT valve was loose and sitting low in its rubber mount/bracket. As such at idle it was knocking onto metal below and sure enough there was a wear mark through the paint down to the metal itself. Amazing that something so small can create such a worrying noise, but also how quickly something like this can be fixed when you go to someone with the right experience!

 

Glad you got it figured out.
What is the MOT valve?

 

MOT Valve is the Purge Valve.

 

Thanks for that.
Do you know what the acronym "MOT" is?

 

I think it means Ministry of Transport (British) as it is an emission requirement and a car will not pass MOT without the Purge Valve or EGR Valve.

 

It is just called a purge valve and the "MOT" written on it is accompanied with an arrow which indicates which port connects to the "motor".

 

Thanks for that, now we know. Why anyone would call it a MOT valve because of that is beyond me. I just looked at mine and it does have MOT -> in white printed on the top of it. Never noticed that before but then again have never had to work on mine. Thanks for sorting that!