Have heard that it is used to either anchor the birds(like making an interesting noise before whacking a delicious bambi),or, it presents a non threatening sound coming out of a very threatening set of teeth and claws while approaching that tasty avian.
Whatever they are trying to do, it is one of the most amusing feline behaviors.
KYPD
Most of our cats sound like machine guns...ackackackackack...when watching birds. One makes a clicking noise. Very odd!