process of elimination.... go to Home Depot, or Lowes, or your garage, and get a cheap extension cord, look for a 14/2 or even a 16/2 label... roughly the same length of what you have connected to your front speaker... cut the plugs off, pull back and separate the 2 wires, and use a wire stripper to expose at least 1/2 to 1 inch of the center wire... and set it aside... this will be your control wire.
Pick one speaker that is 'popping' and one that isn't, front, back, doesn't matter... set them up as a 2 channel, stereo, set , using whatever wires they're currently corrected with... same wires, and unplug everything else... play something , radio, cd, whatever... and see if that speaker still 'pops'. If so, switch the speaker wires... if the 'pop' stays in the same speaker, the problem is either in the speaker, or the receiver... but the wire is fine. If the 'pop' switches sides, then it is the wire...replace the wire connecting the 'popping' speaker with your 'Black and Decker Special' wire, and the popping should disappear. problem solved...bad wire... now swap in the other speakers, one at a time... if it was a bad wire, you should have no more problems. You've just shown both of those wires are good. So now you can determine if one of your speakers is bad. ie, one, or more of them, will pop.
Now let's say the 'pop' stayed on the same side... then wires are fine, and swap it out with another speaker... still 'popping'? then put the 4th speaker in that position...still popping? probably a problem in the receiver... so step back to when you put speaker 3 in... still had a pop, right? and so did the one you just replaced it with.(#4).. but the other one (still #2) didn't.. so now switch the speakers... does the 'pop' switch now? Does the one speaker that you knew wasn't popping before , ummm... popping? or did the pop follow the speaker to the other side? if it did, it's the speaker... if not, it's definitely the receiver.
Wow, that's confusing. But I hope you see what I mean. You can eliminate the wires, one at a time, and you can eliminate the speakers, one at a time. By odds, unlikely more than 1 speaker is faulty, but entirely possible all the wires suck. Having one wire that is different, can show that. And yes, an extension cord is good enough.
I really hope I didn't just confuse you more.
mods, can we have a 'dislike' button ?