Sometimes when car is is high rev's or if the car is warmed up it will sometimes happen on ignition.. I hear a loud noise, very like a dump valve sound that repeats over and over again.. sometimes it might never happen for weeks and then I hear it every 2nd day... I was wondering if somebody knew what it might be.. Car is not a V-tec

That sounds to me like a diverter valve cycling, sometimes referred to as a "gulp" valve because of the sound it makes.

If equipped as part of the emissions system, there are many operating parameters for that component to operated: a few of these are engine coolant temp, intake air temp, etc.

To be on the safe side, make sure the MIL or SES or Check Engine Light comes on when you first start the car, and then goes off and stays off. If you're still concerned there may be a problem, seek a qualified ASE certified shop and have them do a diagnostic test to see if all of the EVAP system components are functioning properly.