I picked up my ML63 a few weeks ago and I've really been enjoying it, its probably the best car I've bought so far. About a week after I started having some strange sounds with the air suspension. It sounded like the air pump was making weird noises. Then I had it where I parked the car and I heard a audible noise for a while after it was locked. From my limited time with the car it should turn off pretty quick. I ended up looking up the relay location and pulling it in case it needed to be "reset". A day or 2 later I came back home parked it, and it did the same thing but for a few minutes, the pump never turned off. I did some research and saw the info about replacing it, so I ordered a OEM new relay. After putting it in, it seems to have solved the problem!! I highly recommend anyone with a old relay to replace it as preventative maintenance. I've seen alot of threads about the pump getting replaced. IMO its probably related to that relay, where it doesn't shut off you walk away from your car for the day after you park it and the pump burns up. It can be just one time that the relay doesn't kick off, and if your pump is even running for 20 minutes or whatever it could break it. The part is only about $20 and its well worth just replacing. Mine appeared to be the original.

 

There's 2 green relays in the main fuse box.
If Green Tyco A 002 542 76 19 then should be replaced with Hella A 002 542 23 19
I relay (1st from left): Starter
G relay (3rd from left): Air suspension compressor