JLCarrera

Good afternoon all — My 991.1 GTS does some kind of short whistling/hissing noise when downshifting if I’m not pressing the gas. If I downshift while my foot is on the gas it doesn’t make the sound. My feeling is that it comes from the symposer at the back — sounds like it is sucking in air… like a quick blow in a glass bottle.

Is this normal? something to have checked? My 981 S doesn’t do it for sure.

thanks!

asellus

PDK or MT?

JLCarrera

PDK!

asellus

Ah, shoot. Not sure. Just guessing here, but perhaps it has something to do with the automatic blipping of the throttle? I have my symposer tubes blocked off so even if my .2 did this, I wouldn't know.

You can test that by having the car in neutral, holding the throttle lightly so it's steady around 3000 rpm (or wherever you are when this happens), then sharply and quickly stomp on the pedal to the floor and immediately let go of it. That's basically what the car is doing to rev match for a downshift, however if you're off throttle it's a smidge different.

JLCarrera

Interestingly, I think the noise is accentuated if the AC is on.

LexVan

What year? If the 3.0 litre turbo, maybe you are hearing the blow-off valve??

JLCarrera

Hi -- it's the 991.1, NA 3.8L

JW911

My car with same engine and transmission definitely does not do that. Strange....

97supratt

Check your air filter housing. If it’s loose or not closed properly it can cause a noise.

JLCarrera

Update -- doesn't do the sound if I put a cloth or otherwise shut the symposer hole at the back of the cabin... I think it's the sound of air being sucked into that hole suddenly. Also note I have my PSE vacuum lines unplugged (and sealed) in case that may be related... any ideas?

asellus

Could be that the symposer is failed or failing, then. No air should actually be moving through that tube, it's a diaphragm that acts as a passive speaker against the intake tract.

PSE vac lines would be unrelated, especially if they are sealed.

JLCarrera

Ugh... annoying. I also wonder if has anything to do with the fact that I removed the sound deafening sponge and the plastic cover at the back of the cabin, precisely to get more of that induction/symposer sound... (but wasn't expecting that tube/whistle sound)

LexVan

JLCarrera, I think we found your whistle. Please see post #11 here:

https://rennlist.com/forums/991/9340...f-991-2-a.html

JLCarrera

Damn that explains why people look at me the way they do.

LexVan

It goes woooo-woooooo, right?