Ok, so I know the hissing thing has been visited before, I searched and coudln't find anything helpful so here goes..

91 z28, 5.7 TPI twin cat.... 86k miles, stock cats

when i'm at half throttle or higher, and get on it, i hear a hissing sound. hissing sound is louder with more throttle.

power does not seem to be affected...the motor will pull to 5000 no problem. G-tech indicates an average of a 14.4 at high 90's mph so it seems like power isn't down.... (stock l98 here).

the engine idles with good vacuum (about 16") and when held at 4000rpm the vacuum goes up to about 19", so this seems to not indicate any restriction in the exhaust.... but the hissing can not be heard when doing this test...i can only hear it under load on the road.

any thoughts? Do the stock GM cats clog up / come apart inside? What else would be causing a loud hissing? I can't imagine the A.I.R. pump could ever pump that much air to make that much noise?

 

My '87 used to make a noise similar to what you describe. I eventualy found out that my two rear most exhaust manifold boltson the driver side had loosened themselves almost all the way out of the holes. This might not be your case, but if it happens a lot worse on a cold engine and goes away slowly as it warms up I would consider it a possibility.