Hiss after shut off, replaced upper plenum
 

Hello all, I am new to this site. I just replaced my upper plenum and gaskets on my 2001 lesabre, 3800 motor. I torqued everything correctly, and everything went back together the way it should. No coolant leaks, etc. Now after turning off the engine, I am hearing a hiss sound for about 5-10 seconds?? Like I said, no apparent leaks or air leaks that I know of. Any thoughs out there? I replaced the upper plenum because I had an external coolant leak, need the throttle body. This took care of it. Any help would be appreciated.