2006 Impala 3.9 engine. When accelerating from a stop I'm getting a clunk sometimes hard (almost feels like some bumped you from behind. It doesn't always do it. 80% of the time it acts just fine. I think it may be a valve sticking in the transmission. Question -- Will this cause damage to thetransmission and how can and how soon should it be repaired? A friend has a ford pick up says it has been doing it for 7 years.