Popping issue with sub

MikeT66

Have a KLH sub power by an Onxyo 501 receiver. Have R50 fronts, R15 surrounds, CSi3 center speaker. Sub plays great but what happens when there is not alot of bass going on in the movie etc. My sub will make a popping sound. When bass is blasting from movie sub works fine. Do I not have in tweek correctly so that when there is little bass needed from sub is pops? Any help you can provide would be appreciated.