Pulleys and tensioners would be the best place to start, IMHO.

Once upon a time, I ignored a slight squealing sound, until the pulley disintegrated and took out my water pump and fan...

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2003 E53 3.0i
DOB 10/15/2002
Sienna red, Savannah Beige Dakota leather
Type FA53, Engine M54B30

2018 G02 30i
DOB 4/21/18
Black Sapphire Metallic / Canberra Beige Vermasca leather
Engine B46O