This is something that I've noticed my car do at idle in park or neutral. There's a faint vibration that runs through a 4-second cycle. That is, goes from no vibration through increase then decrease to no vibration again in a roughly 4-second period, very regularly. Like the engine load increases...that kind of vibration that you feel in the seat but don't see movement on the tach. Any idea what causes that, and should I be concerned. It's a '95 V6 automatic.
