Weird Sound is Getting Worse
 

So my 2000 Boxster S has developed a bizarre sound, that only shows up sometimes, but seems to be getting more frequent and worse. Due to its sporadic nature I can't record it but I think I can better describe.

Under light throttle and light load there is a loud sound from the passenger side that sounds a lot like blowing across the top of a beer bottle. Every once in a while, if the A/C compressor is on, it will do it at idle when the A/C compressor cycles on. It can happen at just about any RPM and the tone doesn't change with engine RPM. If I either lay into the throttle or completely let off the sound stops. Sometimes (like last night) it's a lot more sensitive and seems to do it a lot more, requiring more throttle to make it go away.

Reminds me of my AOS failure. I was convinced I had some pulley or belt noise that would make a squeak squeak noise as the car warmed up and would get worse. It would go away under throttle and come back when coasting or idling. Got to the point one day where it didn't stop even at idle. Turning the a/c on actually made the sound go away at first because of a slight increase in RPM I think.

Check the vacuum on the oil fill cap after the car is running just to see if it could be that. My noise was caused by the vacuum from the AOS failure sucking air through the crank case seal. I couldn't remove my oil cap after I unscrewed it with the car running. Replaced the AOS and it was fine.

Most people that describe the noise you are referring to also mention that it comes from the gas cap or wheel well area. Could be a purge valve from the charcoal canister or a wind noise from the area if it happens at speed.

I have the same noise. I think it has something with the vent below the gas cap. Just been to lazy to check out the vent and see where it runs to. I recently replaced the aos to decrease the white cloud behind my car had no effect on the noise.