Serpentine tensioner is bouncing, anybody have experience with this issue?

[UATahoe]

Noticed last night when i finally got the truck fired up that the tensioner on my serpentine setup is bouncing. Probably bounces up and down about 2 or 3 degrees. The serpentine setup is off of a 96 chevy pickup. All of the front end accessories are brand new(alternator, ps pump, compressor), new tensioner, and new idler pulley. Just curious what could be causing that bounce??? Anybody have any experience with this?

I did have to go with a trial and error approach on finding a belt the right length. Could a belt that is slightly too big be the culprit?