Strange engine knock found

[Tomaz]

I took my 01' 4.4 in for a final checkup before my CPO warranty expired. One of my items was a cold engine knock, I was sure it was a bad lifter. Stevens Creek BMW had the car for a week, cold starting it every morning. They replaced, two chain tensioners, and a belt idler pulley, the noise was still there. Just before BMW was about to start opening the engine to look at internal problems, the service manager suggested removing the serpentine belt that had been replaced a few months ago when I had my water pump replaced. Turns out the belt had bubbles on the bottom of the belt, so when cold, the bubbles would hit the idler pulley causing it to knock. The service manager had never seen this problem before. It was a much easier fix than expected.