I had my hood open, and I noticed that my tensioner arm is bouncing up and down at idle. The belt also rides on the outside of the idler pulley, even if I manually put it on the inside. I've posted about that before and thought it harmless, but now, I've got grooves worn into the smooth side of this (new) belt.

I'll keep digging around under the hood for clues, but could this be caused by a failing tensioner (I hear it'* junk), or by some other pulley binding?

The tensioner could be getting weak. I have a few tensioners if you need one.

My tensioner is under warranty, but I might take you up on that offer if Advance won't give me a different brand.

Last time my tensioner wore out, the belt would loudly squeal. It'* absolutely silent this time. The grooves make me wonder if I've got a tiny pebble or sharp point on the idler or water pump pulleys.

It is possible the tensioner isn't sitting flush. Check the torque of the 3 bolts. One might move a bit. If it'* not flush then the pulley wont be square and the belt will travel wrong.

Upon closer inspection, the tensioner seems to be no cause for alarm. After watching a couple 3800 videos, they all move like that.