I have a Mercedes Benz GL450. When I start the car in the morning it makes a squeaking noise. After driving the car for a few minutes the sound goes away.
It's probably a bad battery (low voltage), bad tensioner pulley (not allowing for proper grip on the alternator), or worn out serpentine belt. This is of course considering, one - it has a serpentine belt, and two - the noise you are talking about is a squeal on start-up (could also be a chirping noise).
When the engine first starts up, it requires a lot of juice from the battery. After it's started, the alternator starts replacing the juice which was used to get the battery back up to snuff. If the belt is slipping for the two reasons it will chirp/squeal. If the battery is getting bad, it could chirp/squeal because the alternator is working overtime.
Must be secondary air injection pump issue. I've had the same squeaky noise for some time and finally got a check engine light come up to indicate it's a secondary air pump. It's easy to replace it yourself, and costs about $100. I got mine here: https://www.amazon.com/gp/product/B00C7P4I0E