I think amrg and 4 speed are on the right track. Figure out anything that could make the engine run less smoothly, or move about more freely (worn out engine mounts). It's a good practice with picking up a used vehicle to treat it to a thorough maintenance as you may not know the history, or have been given an accurate history. New fluids, plugs, belts, cleaning MAF sensor, throttle body. Check engine and diff mounts for wear.