Q: I have a 2005 Ford Escape with 85,000 miles. When idling with the air-conditioning on, I hear a clicking noise from the engine. I noticed that the belt tensioner seems to move back and forth along with the noise. So I changed the serpentine belt, but it didn't make any difference.

A: There are a couple of steps in performing an accurate diagnosis for this type of noise. A required tool is a mechanic's stethoscope, but in a pinch, a long screwdriver held to your ear can accomplish the same thing. While the engine is running and the noise is pronounced, touch the end of the stethoscope to each and every component driven by the serpentine (drive) belt. I probably shouldn't have to mention this, but I will: Do not touch any moving parts. If you're listening to the A/C compressor, listen at the stationary housing, not the pulley spinning at 1000 rpm. Listen carefully. Of course, you're going to hear noise everywhere you go, but you're looking for an exceptionally loud noise that mimics what you're hearing with only your ears. This can be deceiving, and there's no replacement for experience. Noise from one component may transmit through the drive belt (or engine block), and sound like it's coming from somewhere else. Once you narrow the source down to one or even two components, here's the trick: Shut down the engine, remove the drive belt, and listen to each component again, only this time while spinning each component's pulley by hand. Often a worn bearing will be night-and-day louder than the rest.

The noise taking place only with the A/C on was a clue. It's possible the A/C compressor itself is causing the problem, but that's not always the case. When the compressor is engaged, it applies a much heavier load on the drive belt, and oftentimes this load will produce noise from another component. Clue two was the tensioner moving in rhythm to the noise. Drive belt tensioner failure is common, and the belt is very susceptible to producing noise when extra load from the A/C compressor or power steering pump is applied. If the tensioner is the root cause of the problem, you should hear an unmistakable bearing noise while spinning the pulley by hand.


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