I have a pair of old Akai speakers (SW-121A) that I'm running through an amp. One of the speakers makes loud popping noises. I've tried switching the speakers, so I know it's not the amp, and I've tried changing the speaker wire. The popping sound can happen whether or not there is any sound coming through (like if I forget to turn the amp off, sometimes it will surprise me with popping sounds). What could the problem be?