2K Monte Serpentine idler
 

I have tried two different serpentine idlers, they seem to have a slight but noticeable 'knock-knock' with rpm changes. Even at idle it looks like the idler is 'bouncing',,, tried different belt also. Does this mean one of the other pulleys is out of whack,,,, which is causing the idler to be bouncy instead of smoother tension ??? I am sure both idlers were on low end for price,,,,, but will a high priced GM idler act differently ??? 2000, 3.4L .

sounds like you're on the right track with other idlers being worn. As for quality you generally "Get what you pay for". Could also be a bad belt tensioner. I would also give the harmonic balancer pulley a visual for shake/vibration too.

When I said idlers, I meant the entire idler/tensioner part,, I replaced the entire unit twice,,,, and knock returns,,,, my thought is cheap tensioners may not tolerate all other things not being perfect. New Dorman tensioner coming,,, will inspect belt ,,,,,,,,,,,again,,,,,,,,,,, and look at other pulleys for wobble,,, and even debris in grooves,,,,,, more annoying than anything,, but if fixable with new, better parts,,, cool,,,,,,

Good luck with that.
Dorman parts are hit and miss.
I prefer to stick with OEM, ACDelco. A few bucks more, but worth it if it does the job the first time.