I think the most likely cause of the noise is the belt tensioner or belt tensioner shock. It is fairly common for the bushing on the belt tensioner shock to wear out and rattle around and thus make that sound.

I don't think it is very likely to be the water pump making a rattling sound. Don't know the exact location of the water pump on the 4 cylinder, but on the 6 the water pump and belt tensioner are fairly close together.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record