Be that as it may, Carolyn, a cursory search of "popping" sound on this forum will reveal that Shane is far from alone here. My iMac also experiences this, although (I think) it's only when I have the external speakers on.
As with the OP's experience, there is no discernible rhyme or reason. But this has been fairly widely reported, and has been a recurring issue since, I believe, 10.4.