15in kicker L7 noise?

[harley]

i just bought a 08 15in l7 refurbished and i sounds awesome. i have it on a kicker zx 1000 at 2 ohms. however when i have it out of the trunk the sub makes a slight slapping or lite pop when the volume is up which sounds liek its comin from the inside. its not loud but i didnt knw what it might be and if it was gonna cuz a problem. when i have it back in the trunk and have it cranked you cant hear it at all in the car. its like its not doing it at all... so its not affecting anything i just want to make sure its not gonna cuz a serious problem in the future.. any ideas??????? thanx