We briefly discussed this before but I did receive a pair of 21Ipal's that had a ticking sound coming from inside the motor. It was the first pair I ever received. Sounds exactly like you described almost like diesel a engine and got worse with level. Also that pair had a different gasket material and the BL was quite a bit below spec on them. Almost like an old production run or something. My second set did not behave the same way. BL was up where it should be and no ticking sounds. Bennett exchanged the first set of drivers after I sent some video and explained it to him.
I still don't know for sure what the ticking sound is. I've never had an issue with any other B&C woofer.