1998 Taurus SE Clicking Noise

1998 Ford Taurus SE, 3.0L V-6, 131,000 miles

When the car is running there is a fairly loud clicking sound and an accompanying rocking of the car. The clicking sound occurs consistently every 4 seconds when the engine is on. When in the vehicle while the problem is occurring, you can feel a “building” in terms of power, for lack of a better term. When the car is in park or idling at a stop light, along with this “building”, the tachometer rises momentarily from roughly 700-1,000 RPM to 1300-1500 RPM. After a second or so of “building”, a clicking sound occurs, the tachometer instantly drops down to a normal level, and the clicking is strong enough to rock the car back & forth ever so slightly. I’m not sure a passer-by would notice any motion, but you can clearly feel it when sitting with the car in park or at a stoplight with your foot on the brake. The sound is loud enough to be heard standing outside the car or even in the car with the windows up. After it clicks and the car rocks for a moment, it runs normal for 3-4 seconds, and then repeats the cycle of building/releasing.

The clicking sound still occurs when the car is being driven, no matter the speed. It can be heard faintly over traffic noise. However, so far the problem does not seem to effect how the car drives in any fashion as long as the car is actually moving. There is no noticeable roughness, and any rocking is not noticable when in motion.

The issue started maybe 2,000 miles ago, but at the time it would only do it infrequently. Now the noise, rocking, etc, occurs probably 90% of the time I start the vehicle. On the times where the problem pops up, you know it immediately, as it will click/rock once or twice before you even take the car from park to drive. There are still times however where I start the car and start driving, with no issue. There seems to be no rhyme or reason as to when it occurs, regarding weather, how often the car has been driven, or the like. In fact, one day last week, the problem started as soon as I started the car and had continued, but while at a stop light, the noise/motion stopped, and didn’t continue again for the length of the commute. Of course, the next time I started the car, the problem was back.

Any help as to the potential problem is greatly appreciated, thanks.