2003 ML350. I replaced the serpentine belt a few months ago when the engine is cold and in D at idle one of the idler pullies vibrates and makes a loud sound. I thought it might need to wear the belt in a bit but it is still doing that sound. When the engine is hot then it is ok. Anyone know if I got a bad belt or what it could be?

Most likely not the belt but the belt tensioner pulley.

But I did not change anything else. Strange.

Dito on the idler pully. I bet its time has come.

It would be rare but it could be any of the items that the belt drives or the balancer also. However I would look at the tensioner first as I posted earlier. You do not state how many miles are on the vehicle

I have 90,000km on it which is about 60,000miles. I never thought that the pulley might be bad already.

If it is the belt tensioner the fix is quick, easy and inexpensive.

If it is the harmonic balancer it may be covered by a factory recall. My 1998 ML320 with 175,000 miles was recently recalled and replaced by Mercedes. You may want to check into that. But I think your ML350 is not included in this campaign.
With the ease of the job you may want to do the job yourself, as the stealership will empty your pockets and we are not talking about rocket science here.
Good luck
Boston Mark

I have had the ML into the dealership in the last year but they did not tell me about any recalls. But then again sometimes asking specifically is the only way to find out.

I think my next step is to see a mechanic and see what they think of it.

Yeah it is time for you to have someone look at it
Boston Mark