Vibration at 2300 rpm

[greenyone]

I need some help with my 88 944. Recently I've noticed a vibration that you can feel by the floor at 2300, but only on the drivers side of the car. I got out and popped the hood and grabbed the throttle cam and spun it up until I heard the vibration and it sounds like its coming from the valve train. Its wierd because like I said before you can only feel it from the drivers side in the car. I don't know if I have a balance shaft problem or a motor mount problem or if I have a bad lobe on the cam or what. Some help would be much appreciated.