Noisy Engine

[bcald]

I have 2001 E320 with a noise coming from the engine area that I can't pin down. The noise is a deep and low sound until a few seconds later when the car gets rolling does everything go back to quiet. The sound is almost as if I were holding the brakes trying to take off then releasing them after it gets rolling somewhat

My wife's is a 99 ML320 (same engine as mine now) and I don't hear this. I bought this one this past Dec and it's always sounded like this so it's not just somthing that started recently.

Any ideas of even where to start? I am thinking about test driving a similar car at a local auto dealer just to compare. Which cars have this same engine that would be suitable for a test?

Thanks,

Bryant