Hey,
I installed my K&N CAI today. When I checked for leaks after I turn off the ignition it sounds like air is hissing out at the end. Is this normal? If not does anyone know what might be wrong?

 

What you are hearing is compressed air leaking past one or more intake valves that happen to be not quite seated when the engine came to rest after being shut off, thus allowing air that was compressed during a compression stroke pass the partially seated valve and back into the air intake. Yes this is normal. You won't hear it with the stock intake because the entire filter is in a mostly enclosed box which provides enough silencing effect that you don't hear it.
It will stop after a short time as all the gases in the combustion chamber are alllowed to escape and the pressue is releived.
Many vehicles with a CAI or other form of low restriction air filter will do this. My Mustang does it and my Ranger does it also.
Nothing to worry about.

 

I don't pay that much attention everytime I shut the engine off, but I remember hearing that hissing sound too.

 

I hear it too sometimes...good to know it's normal.