Engine noise

[antny]

I searched and found nothing on this but here it goes. Today I go start the car to make sure everything is kosher still after my weekend wrenching. Last time I started the car and ran it the car ran great, no hesitation, idled great purring like a kitty, minus the rattle from the broke exhaust mounts or shields. Anyways today I start it up and it is sounding good, I let it come up to temp and give it a blip of the throttle. There was a sound coming from the engine bay and the car seemed to be moving laterally just a little bit. I look at the belt and looks like the belt under the harmonic balancer is moving in an upward and downward motion and sounding like the belt is touching the other belt. I would say there is a half inch in movement in the belt and it sounds like they are touching. I shut the car down and dont really mess with it anymore because I had to get to class.

Any ideas on where to start? Tighten tensioner? New harmonic balancer? Would a worn engine mount cause so much engine movement during idle? Exhaust didnt sound any differnet and no louder so I dont think it rattled loose or cause a hole to cause any issues 02 misreading and no cel. It almost was like when it got out of close loop it caused this issue. Vacuum leak? I am just trying to get some ideas where to attack first when I track down the issue. Thanks in advance.