I have a 2000 Explorer that I did the front jack chain tensionors on last summer. I was hearing a similar noise so I was looking under the hood and noticed the serpentine belt tension pulley was bouncing quite a bit and that was where the noise is coming from. When I stood and watched the belt for a while it looked like the main crank pulley was out of round and that is what was causing the belt tensionor to bounce. How could that pulley be out of round?

 

i doubt that. take the belt off and play with the tensioner a little and see if it drags or stiff. Start the motor and run 20 seconds while you watch the crank pulley to see if it is o.k.

 

Chances are better than one of the bearings is bad either in the tensioner, an idler pulley, or the alternator. A rough spot in a bearing could cause the belt to bounce and would be much more likely than an out of round crankshaft pulley.

If it were my truck, I'd remove the belt and spin the mentioned pulleys by hand to see if one or more have a rough feel and sound when spinning.

-Rod

 

Slight engine misfire can also cause this problem, Did you replace the tensioner or just the pulleys? could be weak tensioner spring.

 

Having loose bolts on the crank pulley can make it look out of round. Check to see if all pulleys with bolts are tight.

 

Thanks guys, I thought I would be able to check things out this weekend and it just flew by too fast. Maybe tonight I can get the belt off and check things out a little better.