Loud Engine Noise at Start-up
 

Test drove a 2012 ML350 yesterday. Engine made a rather loud, obvious rattling noise when started. It lasted about 5 seconds then smoothed out and quieted. The sales person told us it is something all 2012s do to somehow hasten the warm-up of the engine. Made no sense to me and I find it troubling. Did not try starting another 2012. Any thoughts?

Loud Engine Noise at Startup
 

Thanks ForcedInduction. My problem is it's the Mercedes dealership senior sales person who went on the test drive with us who told me the noise is typical of all 2012s during the first 5 seconds or so of startup. Looks like I won't be buying that car or from that dealership.

Yes there is a fix, Complain to the dealer about rattle noise at start up. There is a service bulletin related to it.

Loud Engine Noise at Startup
 

Here's some new information. Yesterday, I contacted 4 Mercedes dealers in Virginia about the issue. Without hesitation, each service technician (and one service manager) stated that's common with all 2012 MLs and many, if not all, E Class as well. One stated it has something to do with the emissions control. Another said it's common if it has been sitting unused for a few days, but would not happen when driven every day. Another said it would happen when started each morning. Guess it's normal, but do I want to spend 40 grand for a car that sounds like an old beater for 5 seconds when I start it? (My wife really wants one of these so I guess I may be stuck.)

The noise is caused by the secondary chain tensioners which don't hold pressure. There is one tensioner under each cam cover at the front of the engine. Document # LI05.10-P-056435.
Only for engines 276 , 350 and 278, 550 engines. And AMG engines 157..

Engine Noise in ML350
 

Thank you Chassis221. My apologies in advance for this rather dimwitted response. Since I don't understand chain tensioners and I'm not sure whether it's a defect that they don't hold pressure, I'm still wondering whether I should buy the vehicle. The bulletin attachment is too small for me to read when I opened it on my computer and I'm not sure I understand the overall meaning of your response. Let's see if I have it correct. Engine 276 in the ML350 and engine 278 in the ML550 will make this noise and it's either put up with it or don't by the vehicle. If that is correct, here's the really dumb question. Were there engines other than the 276 made for the ML350? In other words, if I shop around can I find one with a different engine that will not make this noise and, if so, how would I identify it?
Again, thank you for your patience.

Try the belt tensioner, test bearing with bench with belting on. Otherwise check the belting for crack, cut/torn.