TheBruce

No, as mine is a base C2.

Best explanation was the 91 octane but ur comment about exhaust pipes clanking made me want to reinvestigate more.

Petza914

I may have changed my post before you replied as I thought the airbox resonator was only on the S cars, but believe it's on all of them, so your car being a base car isn't exclusionary anymore.

Factory airbox in yours, right?

TheBruce

Yes, factory. But I could try putting my resonator plug back in to see if that has any effect.

rileyracing1

My 997.1 C4s does the same thing ... I am running now 94 instead of 91 and it's less and here in Toronto it is getting cooler which also helps ... I have also altered my driving habits as well, to virtually eliminate the dreaded low rpm / throttle blip ping as I call it.

lowbee

I heard it whenever I am near 2700 rpm and changing my throttle input from very light to moderate. I always thought it was a valve in the intake opening or closing so I never worry it.

Edcantera

I have the same issue when blipping the throttle from a low rpm. I have the factory air box with the Helmholz capped off using the Fabspeed kit:

https://www.pelicanparts.com/More_In...CABEgJAi_D_BwE

I’ve listened carefully when rev matching from higher rpm and have not heard the “knock” sound. Still very curious to know what the actual cause is. I don’t think that it’s related to the factory air box, or at least not the Helmholz.

Fuggit

Have you checked your engine mounts? They could also create the noise you mentioned.

lowbee

For me it is more of a "click" or "tick" sound than a "knock" sound...

TheBruce

I did a low rpm blip the other day just to see if I could better better describe the noise. Its not a single noise, its 2-3 knocks, that sounds like the engine is stumbling. It does sound metallic, which is why I was originally worried is was chains slapping against the tensioner. But im pretty sure its just engine knock. I hear it clearly with the driver window open. It sounds almost like its coming from right under me near the transmission, but I assume thats just reverberation.

I am going to have one of kids blip the throttle in the garage to see if I can record it. Im also interested to know if it only happens when the engine is hot.

seewong

I intermittently get this noise too, only when the engine is hot and I am blipping the throttle. It occurs most often when exiting the freeway (idling to a red light) and the light turns green before I get to it. As I blip the throttle sometimes it will knock/crackle. I always just assumed it was normal for a car with Fisters...

lmaternick

I get the same noise under the same conditions. I have no rattles and rennline semi solid mounts.IMO it is exactly what a previous poster suspected.......cam chains getting some backlash. Mine at least sounds nothing like detonation. It sounds exactly like chains being slapped against something hard or metallic. When I had a bad misfire issue this past spring and it wouldn’t even idle it was accompanied by the same sound as if it were bucking and slinging chains all over the place. These engines don’t like to be gassed until above 3k. So that’s where I drive it.

TheBruce

Ive been investigating this further. Nothing definitive yet but it doesnt seem to be linked to engine temp. My newest theory is its linked to deceleration. Thats the only time I get it. Ive tested in my garage and cant reproduce. Ive tested while coasting and cant reproduce. Ive tested at colder temps or hotter temps and cant reproduce. The only time I've been able to reproduce is under braking coming to a stoplight.

LucV

I've noticed this as well. When in 2nd gear and around 2700 RPM, I'll hear some knocking/rattling sound for less than a second if I hit the throttle a bit too quickly. I don't hear it if go smoothly. I never hear the noise at other RPMs.

I do sometimes hear the same noise when lifting the throttle around the same 2700 RPM and still in 2nd gear.

I run 91 Shell Nitro-V+ (or whatever it's called) as this is the highest octane we can get here if you don't want ethanol in your gas.