Knocking Sound and part ID
 

Wife's 2012 ML-350 with the OM642 did something really strange. On first startup today it started knocking, with a moderately heavy knock you could clearly hear over the normal diesel sound. I had her pop the hood and I took off the plastic engine cover. I quickly isolated the sound to a silver (aluminum) colored part on the top left of the engine (see attached pictures, the pencil is pointing to the part) I put my hand on the part and could feel it rapping internally in time with the knocking sound.

After the engine ran about 3 minutes the rapping and knocking sound became intermittent, and 5 minutes after starting the sound and internal rapping were both gone.

Can somebody ID the part in question, and tell me how difficult and/or expensive it is going to be to replace it if it keeps acting up?

Many thanks

Doing some research, it appears the part is the vacuum pump.....can anyone confirm. I'll ask the question in the ML section also. Thanks

That is indeed a vacuum pump, however are you sure that is making the noise? Those pump are very simple in design and rarely fails with low mileage.

If you want to check the pump noise better, carefully (oil will filing out from the timing chain) with the engine running open the oil filler cap, you should hear the vacuum pump deep tapping away (its normal), however should not knock as you describe.

Heres a normal vacuum pump sound with the oil cap open and engine running
https://www.youtube.com/watch?v=3cxAyOa0m94

Yep, that is what is definitely making the noise.......and I was able to feel it rapping when I put my hand on the pump while it was running. Funny thing is that with the hood of the car closed it sounds almost like the deep sound in your video, except it was a sharper and louder knock. The wife also said earlier today that the HVAC system in the car was acting a little flaky at times and apparently that is vacuum controlled.

when you have a vacuum leak they will make the noise you are describing. What did you find?

It was indeed a failing vacuum pump. Put a new pump on (fairly simple) and the issue went away.