"bling" - flight deck often utilise the momentary cycling of the seatbelt signs as a means of communication with the cabin crew. Generally, the ping you here shrtly after take off is to signal that the cc are free to move around. In my company we normally place the seatbelt signs on passing 15000 feet in the descent. If they are already on prior to that due to turbulence then a cycle of that switch again represents a signal that we are passing that altitude on the approach and start to prepare the cabin accordingly.
Alternatively maybe someone's just impatient for a drink???
