I have a hissing sound from around the floor that robs a small amount of power but stops above 4000 or so.

Under hard acceleration it doesn't hiss, but happens under medium acceleration until like I said around 4k then goes away again and you feel it run better.

sounds like it's coming from in around the inside passenger floor/firewall

 

Look at the IC coupler that goes to the Y pipe. Take it off ...I bet it has a razor cut in it.

 

it would be nice if you included a sound clip...

 

It could be the hard line for the manifold to the cat. Its for air injection. I think the air pump turns off at 3 or 4k so is consistant with what you say. It attaches to the manifold by the firewall and runs beneath the passenger floor board to the cat.

 

thanks guys!

 

id say airpump. mine does the same thing. but my air pump has a really loud clunk when the clutch releases and grabs, so i always know when the air pump is running =( i also hear the hissing sound...i wouldnt worry about it

 

Quite to the contrary, I'd say worry about it. If you still have the stock cat, the air pump is necessary to keep the cat from clogging. I'd look into finding/fixing that leak.