[blinkwatt]

I ran home the other day and left the car running in the driveway. My driveway has a steep incline. After running back out to the car to put stuff in the rear trunk I heard a knock like sound. I think it was either coming from the cats or tranny. As soon as I brought the car on a flat surface the sound went away and I had to really try and listen in to even hear a faint trace of it. My car was idling on the incline for about 2 minutes before I even heard it and this was the first time I've heard something like this from my car. I've had the rear end backed up on ramps and had the car running for 10-15 minutes and haven't heard anything unusual before.

Anyone have a clue what it could have been? My car hasn't leaked anything from it and as been running strong and good lately.