My customers car is a 97 Chevy Cavalier, automatic.My customers recently had the serpentine belt, spark plugs, oil filter, etc. replaced, about 3000 miles ago. Yesterday it started acting up. The engine seems to rock intermittently when it first starts up, and when shifting between the lower gears MB STAR C5, there seems to be a power loss, and occasionally, my customers can hear/feel a knock under my customers feet in the center of the vehicle. What's the prognosis?