I did just like the instructions. Capped off everything and now it makes this sound when I start the car cold or if I get on it. No sound at idle or cruising. It sounds like a hissing sound and I am sure its not a vacuum leak because I checked all my lines.
The only thing I can think of is I took out the IAC screw device because I was trying to get my distributor bolt loose for something else I was doing. I put it back though and disconnected the battery. could that be the problem?

 

Both my old z28 and my truck do the same thing. Your car has always made these noises, but until you did these mods they were muffled by an enclosed air cleaner. The same way sound travels through your exhaust, engine noise can travel through your intake. When you open your throttle blades its the same as opening a cut out in your exhaust. I'm not sure about the hissing on start up but i've always figured it was the intitial rush of air being pulled through the closed throttle blades. Anyway what your hearing is normal.

 

The "hissing" sound you hear during cold start or during wide open throttle is a very good indication that the motor is getting all the air it needs.Open element filters allow more air in at a faster rate than the stock fully enclosed units.This rush of air is what you hear..during cold start the iac motor will rev the motor ad add alot of fuel to warm up the motor and slowly bring the idle down while slowly closing the throttle blade.Also during wide open throttle..your demanding everything at max optimal potential..which is alot of air..fuel and spark..this means..as much air as the motor can breathe in through the intake will hiss very loud..and breathe out trhough the exhuast which will be loud. Hope this makes sense..nothing is wrong..other than your going to be trigger happy having a lil bit more hp from a better breathing motor lol

 

Yes that's a really nice sound if you ask me. Ive got the 14 x 3in. open element with the x-stream filter top and cowl hood and man you can really hear that thing suck air.

 

Cool. Thanks for clearing that up guys. The sound is deffinatly different than my stock setup. Much more "performance" sounding now. I have a 80 series flowmaster as well to help with the sound. Would like to do some headers next. Will see how that goes.