I also noticed on one of mine there was excess clear resin on the backside of the cone near the surround edge and at near maximum excursion it was making contact with the spider, making a sort of clicking or tapping sound. Once I removed it the sound was gone. Another thing to check is to look on the underside of the surround, the entire perimeter, for glue drips that have hardened. That will also make a clicking type of sound. Be careful when removing it as you could tear the surround. The limiting factor for this driver seems to be the backside of the cone contacting the spider at max excursion.