my car makes a pretty loud popping sound when i'm decelerating. it def. the exhaust, and i just put in a k&n filter (the panel filter not the cone). could this be the cause of that sound? i know bikes, and if they work the same that would mean it's running lean, which makes sense because the k&n increases air flow.

 

You say Its the exaust? When the cat on my car went, It left me with a god awful "marble in a coffie can" sound when decelerating. I can't comment about the K&N making it happin, butt when it happend to mine, One day it was fine, the next day It sounded like SH**

 

when you replaced your cat did you pay a grand for the audi one or did you use an aftermarket? my left cat is bad according to the old repair invoices i have (i just bought the car about a month ago), it says internal failure, so that's probably the rattling i here when i'm decelerating. i was only thinking about the k&n because i'm used to bikes, but fuel injection is completely different, so it shouldn't be an issue. i'm pretty sure it's the cat from what you've said. i have no CELs or smells so i'm not in a hurry to change it. especially at that price. i've heard the aftermarkets are no good and can cause false CELs. any ideas?