Hissing From Engine Compartment

[RaisedOnPorsches]

Greetings,

On the way to work today I noticed a soft hissing sound coming from the engine compartment of my '99 Boxster. It sounded like the hiss of steam, faint but steady. When I got to work I immediately stripped down the car to access the top of the engine while leaving the car idling. I didn't notice anything. The hissing only seems to be present when in motion. As soon as I got to work and parked the car, the sound was gone.

Leaky vacuum tube somewhere? Another symptom of failing AOS? (I am planning to replace this Saturday) Coolant leak? Anyone run into this issue before? Have an suggestions? First thing I'll do this afternoon when it is time to drive off is check the rear tires to be certain it isn't a puncture. That said, I am almost certain the sound was internal and from the engine compartment. I've only heard the sound when in the cockpit, but I had a passenger who noticed the sound too so I know I'm not going crazy/paranoid.

Thanks,

-Jeremy

[larez2]

It could be your expansion tank or expansion tank cap, especially true if you still have the original one. It may only be leaking while under load (ie accelerating) so that's why you don't here it when stopped. Check under the carpet to see if there is any coolant pooling underneath it. Also remember that it is a pressurized system so be careful.

Good Luck,

larez2

[fasterjames]

When you heard it were you driving by a building or a guard rail? I have a mysterious noise too -sounds kind of like exhaust wanting to come out but being blocked. More of a pressured whooshing than a hissing but perhaps we are hearing the same. I don't hear it at idle either or unless I am passing by something and getting reverberation from it. Does this describe what you are experiencing?

[RaisedOnPorsches]

When you heard it were you driving by a building or a guard rail? I have a mysterious noise too -sounds kind of like exhaust wanting to come out but being blocked. More of a pressured whooshing than a hissing but perhaps we are hearing the same. I don't hear it at idle either or unless I am passing by something and getting reverberation from it. Does this describe what you are experiencing?

I could hear the sound while the car was moving. Didn't matter if there was a wall or such next to me to bounce the sound back. The sound was more internal that external. As of last weekend I've gone ahead and replaced the AOS anyhow (due to other symptoms). I also noticed that I still have the original coolant cap and there is a little bit of white crust around the port. Possibly that was the cause. I'll be swapping out that coolant cap ASAP. I should have done it last weekend, but after doing an oil change, spark plug change, air & pollen filter change, and AOS replacement I lost track of that last item. If I'm ambitious enough I'll be cleaning my throttle body this weekend too for good measure.

Good luck diagnosing your sound.

-Jeremy