Audio Systems: speaker pop/pulsing, ar speakers, gut feeling

Question
I set up an older amp with two AR speakers that I rebuilt with new cones, etc. One of the speakers sometimes begins pulsing or popping. I can see the cones stressing as if heavy drumming was happening.  I looks as if I did a good job of gluing in the cones, etc. so I can't figure it out.  Any clue?  Could it be my old Proton amp giving up.  I'll add that it's not all the time that it happens.

Thanks very much.  Ken

Answer
One way to test this would be to swap the left and right speakers, and see if it happens again on the same side. If it's always coming from the left channel (for example), I'd suspect the amp. If it "follows" the speaker to the other channel, it's the speaker.

My personal gut feeling from what you've described would be an issue with your amp.