My money is on the A319 speed brakes being the source of the noise you can hear.
On approach to STN from the west, we are often kept a little high to cross over the top of LTN arrivals from the east and STN departures. If that's the case, then northeast of Luton out come the speed brakes to regain our approach profile.
The faster we're going, the louder they seem to be!
DH