It is officially called a noise encapsulation panel. Among the various things it does is provide sound insulation, improve aerodynamics, keep the serpentine belt as well as the rest of the engine dry and help keep things clean.

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Ali Al-Chalabi

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