its impossible to say by what he has said so far.. you have no idea what kind of pop, how often, or when it happens..
not one at a time as they don't sell em like that but in pairs yes.. but not sure they are blown yet so lets figure this out
when is the popping happening..
constantly while playing music, randomly throughout music, when turning on/off the car?
what kind of popping is it? a single pop like a quick mic tap, real fast popping like someone dropped a mic?
frequency.. Is it a high frequency pop(almost like a crackle), or is it a low pop?
is it coming from multiple speakers or just one? is it coming from the speaker and tweeter in the sets or just one of them?
just a couple questions.. please answer them to help figure this out... I have a few more follow up questions and will ask them after you answer