2005 B6 4.2L
Question, after a week of sitting in the garage, I go to start it I hear a thumping noise coming from the underside like it might be from the cats, the CE light was flashing too, after a few seconds to a minute it stops and runs smooth, any ideas?

how many times does it thump and how loud is it?

@10 times in rapid secession it's as loud enough to startle me, very similar toif a basketball dribbling under the car.

it only happens when I have not driven it in a week or so...

I would definitely say it's from the engine and nothing like the drive train. But it's strange that it only happens after not driving it for a while. Could it possibly be backfire or do you think the sound is more mechanical? And is it very sequential and like a consistant rythem or more sporadic?

It sounds like a loose muffler bumping the underside of the car, but the car is not started yet, it happens just after I turn the key on and before turning the motor over...

Sounds like you are having missfires. Especially if your CEL is flashing. How long has it been since you have replaced your spark plugs? Also have it scanned and report the codes back to us here.

Josh

I bought it CPO last December with 36,000 miles on it, so it's never been to the dealer with any enging related problems. I assumed that they changedthose before selling the car, but I highly doupbt it! I have noticed a drop in performance and power so that might be it.Weird that after drving it the lastfew days all of these problems (except the loss in performance) have vanished. It's going to the dealernext weekto check the problems I'll get the codes, and let you all know what transpires!

Thanks,

Phil...

Just take it to an autozone or shucks or whatever you have around you and have them scan it for free and post those. It might be an easy fix.

josh