As you can imagine trying to diagnose a sound over the web is nearly impossible. It has to be done in person. Given the several different causes; belts, pulley's, alternator, water pump, fan, etc.. Any suggestion would be a shot in the dark. Goodluck I hope you get this resolved.

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2005 BMW X5 4.4 Sport Upgrades:

Dinan Cold Air Intake
Dinan Stage II Software
Akebono Euro Cermaic Pads
Brembo Rotors
Titan Silver Metal Gauge Rings
DICE IPOD Module
Hardwired Passport 9500ix Radar/Laser Detector