I just bought a 2000 Chevy s10. It shakes when accelerating moderately in gears 3-5 (manual transmission). If I accelerate very gently, it doesn't happen. There is also a knocking noise every 1-2 seconds when it is shaking (it sounds like it's coming from the lower driver's side of the engine bay, but I can't be sure because it only happens when the vehicle is moving).
It accelerates fine in 1st and 2nd, so I don't think the transmission is slipping. What could be causing this problem?