more slant problems

well i took it for a test drive today and it went great, no stalling whatsoever :D i guess the timing did the Job. one thing that did happen though that actually has been happening for a while on and off is that after i take it on a drive, when i turn the car off it makes a sound almost like steam shooting out of something.... the car doesn't have an overflow bottle yet so i was wondering if that could be the cause of the noise? there is a hose but no bottle. it hasnt seemed to affect drive-ability in the past but it is a bit annoying and i would like to figure out what it is.