a hissing noise from the intercooler could mean a leak like a weld or one of the fins cracked under boost....see if you can locate the source of the noise on the IC.
if you suspect a leak but cant track it down over engine noise you can even take it off the car, just seal one end with your hand and blow into it to see if it leaks.
If your IC has a leak then thats your loss of boost...
Just pray that nothing foreign has entered your turbo and has damaged your turbine or compressor, worst case scenario is something was sucked into your intake, damaged your compressor and a part of the compressor itself is lodged in your IC either causing a leak of its own or a hissing noise by obstructing one of the fins.