THUNK upon acceleration can be unnerving. It can also foreshadow an expensive fix if not diagnosed and corrected before serious damage is done.
One of the most likely places to look for throttle-induced noises is at the joints between the transmission and wheels. U-joints and CV-joints have finite service lives and are designed as weak links in the driveline-they're cheaper and easier to replace than gear-train parts.
Diagnosis
If the driveline joints pass muster and the problem still exists, zero in on the motor mounts. These mounts are designed to flex and absorb a certain amount of vibration inherent in any running engine. Over time, the mount's flexible part figuratively becomes arthritic, losing its absorptive its. In more extreme cases, the mount becomes fractured. This can allow the engine to literally clunk around under the hood, rotating under acceleration and other weight-transfer conditions. This engine movement transfers additional stress downstream to other driveline components. In worst-case scenarios, abrupt acceleration or braking can cause worn-out mounts to fail, sending the engine into the radiator or other places. Fan clicks against the radiator shroud is another telltale sign of bad motor mounts.
Motor-mount inspection isn't always easy. The typical mount combines metal mounting surfaces with hard rubber or polyurethane sandwiched in between. This flexible interior substance degrades over age, but even cracked rubber or urethane isn't always detectable to the naked eye. (Black rubber in concealed areas often doesn't reveal the true state of its health.)
To check motor mounts, consult a service manual for proper procedures for your vehicle. (Obviously, transversely mounted engines are secured differently than longitudinal ones.) The procedure often requires slightly elevating the engine to take its weight off the mounts, something that shouldn't be attempted by the ill-equipped or faint of heart.
Installation Notes
> Original-equipment motor mounts usually have a hard-rubber component. For popular/enthusiast-oriented cars and trucks, the aftermarket offers harder-than-rubber polyurethane-core mounts. In general, urethane is more durable than rubber, but it tends to be stiffer, thus absorbing less vibration. Also, some companies manufacture urethane mounts in a variety of colors.
> The motor-mount replacement shown here is on a front-wheel-drive car with little free space in the engine compartment. A hydraulic lift was used to aid photography as well as the actual job. The jackstands-and-creeper method is possible, but is probably more time-consuming.
> Even if only one mount is damaged, consider replacing both at the same time. Lifting the engine's weight off its mounts is the most time-consuming part, so it should only take a few more minutes to replace the other side.
> Always remove the battery's negative cable prior to lifting and securing the vehicle.
> Barring accidents, motor mounts generally have a long service life. Replacing bad ones as soon as possible will limit damage to other components. In many cases, a fresh set of motor mounts will outlast the rest of the car.
> On OE-style mounts, applying a rubber preservative will extend mount life even further.
