Just did this today
My car has 168,000 miles and is a 2001 E46 ZHP 330i.
I have had a crazy whistle from my engine ever since I took it to the auto mechanic for an air leak I gave up on. My engine runs great. But gas mileage is very poor and I get a LOUD whistle until the engine is completely warm. It whistles at idle, when accelerating and maintaining speed. I just did this "Intake Boot" write-up DIY today. Used a factory boot. All went very smooth as I read this FANTASTIC write-up before proceeding. The whistle is still there. I see many references to possible pulley/tensioner being the source of the whistle. Are these parts expensive? I consider myself really good at DIY, but no master as I need to do research before I take on any task beyond brakes & oil. Would it make sense to just replace these parts, anyways? I plan to keep my car as long as possible... My goal is 1,000,000 miles. Any feedback is appreciated.