Pelican Parts Forums - Hissing noise when accelerating

My 2002 Boxster S makes a hissing/"psssst" noise from the top of the engine area that sounds almost like a relief valve or something blowing off air when I first accelerate and when accelerating every time I change gears. It's most noticeable if I blip the throttle when the engine is idling.

I just bought the car a week ago and have replaced both the water pump and AOS (they were both failing when I got the car). So I don't think this has anything to do with the AOS (new one seems to be fine... no more oil smoke and the oil cap test is all good).

I also took the throttle body out and cleaned it to get rid of a little idle hunting it was doing (lots of oil in there from the failing AOS). I made sure all the vacuum hoses I know of connecting to the intake manifold were tight and I don't think there are any leaks there. Not to say it isn't possible there's a leak in a hose somewhere though...

I found a video online of a 987 driving around with the engine cover off and it makes the exact same noise. Can someone take a listen and let me know if this is normal or if I should be concerned? It's hard to hear the noise at all when my engine cover and carpeting are in... but definitely noticeable with just the engine cover installed and the carpeting out.

Here is the video. You can hear the noise distinctly around 20 seconds and 30 seconds in (as well as pretty much every time he shifts).

YouTube - ‪Porsche Boxter 987 engine sound driving‬&rlm;