intake noise could be it.

You would know if it was exhaust because it wouldnt be like a hissing. It would make your exhaust sound as if you dont have one.

Maybe you have a small leak in a hose somewhere and it puts enough force on it when you are on it hard that you hear it then.

Or you could be hearing alternator whine through your stereo. That can sound like a hissing. If it is from your stereo go and check the ground connection on your amp and your head unit. I think this may be it since it is when you are onit hard and when you hit a bump. A loose ground connection will do that.