If it comes off the intercooler and is around 60mm in diameter, then it is an air hose most likely. It sounds like the coupler between the y-pipe and the crossover pipe that leads to the intercooler.
It is not a one-way valve.
There should be two hose clamps on it (one upper, one lower).
If you are hearing air hissing then you need to check for loose vacuum lines in the area.
Last edited by wrankin; 09-18-04 at 03:23 PM.