I had an issue with my tensioner. I watched a Utube video on it and will try and find it again..... but the issue was how the spring sat on the mount. The spring is suppose to ride in a groove and it tends to slips out of the groove. if you look closely at the bottom of the spring you should see where it should sit on the mount.
Are you dealing with an old used (stretched, glazed, all cracked up on the underside) belt or a new belt? Are all your accessories turning freely (as in Fan clutch, Alt, smog pump, A/C clutch, Water pump, PS pump and anything else I might have forgotten)? They make a plastic wheel version and a metal wheel version!
I'm on my second tensioner it is the metal version and all is quiet here, with 312K miles on the engine.