Code p0456

i'm new here and i have a Eclipse 2003 gts wich has a code P0456 i cannot get red of it and it comes on only early morning when it's cold outside and then i cleared it and then don't come until may be the next day morning if it's cold outside if it comes back. I changed the gas cap it didn't fix it. If anybody know how to fix it or had the same code and were able to fix it please, let me know, I would appreciate..please help me ...S.O.S.

P0456= evap control,very small leak detected. The evaporative emission control system has a very small leak. Needs to be taken to a competent repair shop for system smoke detection. Any collision damage to car?

DdeltaP, first, thank you for replying for my tread . anything i could do to fix it because i went under car to check all the fuel lines and conaster and they look ok however, i didn't had time to drop the gas tank to see may be the fuel pup or some hose loose there. I know also that i hear some hising noise from the upper intake manifold from the left side just a little hissing noise, may be that's tha problem..i don't know... let me know..

collisions: yes i checked the car fax report and the car has tree accindents in the past ( long time ago)
that hissing noise from the left side of the upper intake manifold is that normal and purg selonoid make some noise too

anybody has any ideal what it could be the problem with code p0456? i nedd help with this....