What?

'Loose' says something about a specially ribbed belt quiet , and you say something about the idler pulley riding on the back of the belt? If the ribs ride over the idler, wouldnt that be noisy?
Surely the toothed side isnt ribbed? is it?

You guys are either both talkin 2 different things, or someone needs to add some details here.

Please everybody stop making so much sense and being so informative...