Hi, haven't been around in awhile. Did some searching, but haven't nailed anything down yet. Basically at higher rpm and load (accelerating to get on freeway, passing, etc) I get a hissing noise from what sounds like the right side of the engine and a loss of power it seems. Under average acceleration from a stop, there's a point where the exhaust note gets quieter and the hissing starts, but not too loud. Goes back to normal after acceleration and come to a stop again.
I'm trying to figure out what the vacuum regulator and vacuum switch do exactly. Does something there open possibly from the ecm when it see's a specific load based on maf and tps input? Also thinking about getting a bunch of plugs and blocking off the line to the throttle body and evap cannister, wherever vacuum is pulled from, and see if the noise goes away.
No codes. Spraying with brake clean does nothing either when idling.
89 2.8 automatic system.

 

I guess I fixed it. On the way home the other day it got really bad, figured I would surely be able to pinpoint it when I got home, went to pass someone and it's completely gutless. I stood on it, and after a second or two, it instantly cleared up and seems to run fine now. Cat stopped rattling too. Weird.

 

Before symptoms said clogged cat. Current symptoms sound like the inside blew out the cat.
Pure guessing based on a couple of key words in your description. Actual investigation should be done.
You might have an empty cat converter and the insides have rattled down farther in the exhaust.

 

That's what I'm thinking. Cat started rattling about a month or 2 ago, but was replaced 6 months ago. I don't know what brand shop put in. It does seem a tad louder now than before the hiss/power loss started. Anyway, doing a mileage test atm. Odometer stopped working last week, and had to get odometer motor out of my other free firebird to make it work. It had a gear that was stuck and broke off a couple teeth when I tried to turn the motor by hand. Now I need a gear and motor to make it drivable once I get the windows to work.

 

A lot of guys recommend smoke test to locate vac leaks. never done one myself, but it sounds promising.