Hard metallic knock when starting to drive

I have a 1998 Mercedes ML. In the past few weeks, there is quite a loud metallic knock when I start driving. It does not happen if I take off real slowly. What could be the cause?

The first thing that I would check is for a bad u-joint on the drive shaft.
What you describe is a classic symptom of a failing u-joint, and on a 14 year old vehicle, it would not be unusual to have a u-joint (or two) worn out.

Is it a single knock or a repetative knocking sound?
I agree with VDC if it’s a single knock, but if it’s repetative that would suggest other possibilities.

I’m in agreement about a driveshaft problem and if not a U-joint it could be a center carrier gone bad.

This specs out as 4WD, 4 wheel independent suspension. So many CV joints (perhaps double cardon joints in the rear??). So many possibilities.

Thanks guys. I’ll have it checked out. Do I assume its not going to break down on me sometime soon.?

You should not assume that. A faulty CV joint, U-joint, or center carrier (assuming this problem is one of those) can come apart at any time.
I would think that if the problem were drivenline related though that there should be some more symptoms such as a vibration, shudder, buzz, etc.

I agree. There are too many possibilities here that could come apart on the road and cause total loss of control. Assume nothing. Your safety may depend on it.