J. Hidalgo is right.
That serpentine belt tensioner can make a racket if it's going bad.
There is also an idler arm pulley that this serpentine belt goes around as it loops around the water pump. The bearing in this pulley can also make a rattle noise.
Last, your water pump could also make a noise if it's going bad, but you would also be likely to see a coolant leak or be losing coolant.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".