Ok guys, I'd like to get your opinions on what this sound is before I take it in the get it looked at. It's a hissing sound that seems to be coming from the engine. After a short drive, at idle, you could hear a very discrete hissing sound. I don't recall hearing such a sound before. No aftermarket intake, just the stock airbox. Any ideas?

Also, is it normal for the clutch to be vibrating when going highway speed?

 

Vacuum leak that might the noise and idle issues

 

Take your cat out of the airbox.

I decided to be a little more helpful so...

IIRC a lot of the S2000's have some noise from the airbox. Is there any other issues with the car currently? Lumpy/inconsistent idle? Power loss or engine hesitation? Anything besides the sound of moving air?