I just got my timing belt and balance shaft belt replaced today. The car runs fine, except for when I decelerate, when it gets to about 1000 RPMs it gets a slight vibration, only for 2-3 secs, then it's gone. We tore it back down after the test drive and re-checked the timing of the timing belt and the balance shaft belt. Both were dead on. Any idea's where the vibration could be coming from? It's not a real noticble vibration, but I know it wasn't there prior to the belts being installed...

 

I think it has something to do with my idle. I've been watching the tach and when it starts the vibration, the tack starts to moving between 1200 RPMs and 800 RPMs. Once the RPMs settle, the vibration is gone. I'm wondering if the what ever controls the idle is going bad...