What causes a serpentine belt that has been changed three times to make a chirping squeaking sound that is louder than the one before on a 2000 Ford Taurus when the last one was a Ford belt?

And yet one more possibility, based on the use of the Word "chirping" - the camshaft position sensor / synchronizer.

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AnswerSounds to me like you have one of two possibilities. One is that you have either a bad belt tensioner or a pully in the drivebelt system. I would spray some silicone spray on the belt and see if the noise stops temporarily. If it does, put a new tensioner on, the tensioner is allowing the belt to slip. If it doesn't stop, then check the other pullys. Spin each one and esp check play and alignment of the pullys. The second possiblity is that you don't have a belt problem at all. Are you sure the noise is coming from the front of the engine? Double check, because if you have a 3.0 twelve valve engine, and the noise is coming from the rear of the engine around the thermostat housing area, then you have a bad cam synchronizer. They sound a lot like a belt squeeking, and we get people in all the time at my shop thinking they have a belt noise, and they really have the other problem, so check. If you have a 24 valve engine however, then the cam synchronizer is not a possibilty since it has a completely different set up.

I have just thought of a third possibility. If you are positive that it is the belt, check also to see if there is a coolant or oil leak on the front of the engine in the belt area that could be contaminating the belt. The coolant jug especially on those things love to crack on the bottom and cause coolant to leak onto the belt. This would definitely cause a belt to chirp.