Noise from engine bay

[thebairdybunch]

I have this strange sound - A vibration noise - the sort of sound you would get when the exhaust blows - Its quite loud and coming from the back end -- its not low pitch sort of a metalic rattle sound ..

However . I

1. Only get it in gear and when I floor the car -

2. It lasts just for about a second or two until the car starts to move say past 20mph

3 It seems to come more from the right hand side read

I dont get the rattle when I rev it in neutral nothing - - I only happens from stationary when I floor it

I fitted a K&N filter just after christmas but didnt notice any rattle - I checked it today and nothing is loose any suggestions welcome

1. Heat exchanger ? - If so how come it only makes a noise in drive and when its floored

2. Transmissions ?

Suggestions welcome -

Car is a 00 C2 996 CABRIOLET

[White987S]

Bad cats?

[CosmosC4S]

My guess would be the cats as well.

[p0rsch3]

i have an almost identical problem. I thought it was mufflers so changed them but it's still there. it does indeed only make the vibrating rattle when nder hard accell taking off in 1st.

if I punch it in any other gear or lightly rev in neutral, no noise.

did you find out if it was the cats? how do you check if your cats are bad?

[p0rsch3]

update- I noticed that the noise only occurs when the car is warmed up. If I floor it while not completely up to temp, there is no noise. Any thoughts?

[996TT2001]

Any new ideas?