Hi Landroger...
I don't go along with 'dany4skin' at all...the use of speedbrakes anywhere near that sector of the flight would be very unusual indeed!
My best guess would be that you are near the spot where, as the airplane is slowing on the localizer (electronic signal that is an extension of the runway centerline), probably with 5 flap set. Then the intercept with the glideslope is made (glideslope is another electronic signal that tells the autopilot to start/maintain a 3 degree slope towards the runway threshhold, arriving there at about 50 feet above), followed right there with selection of 15 flap and gear down.
This large amount of increased drag (15 flap and gear down) then forces the auotothrottle system to increase thrust to maintain the desired speed...what you are hearing is that initial 'growling' windup of the engines from their low(ish) setting to something quite a bit higher.
Sorry if I have over-simplified this, but I don't know how familiar you are with these basic maneuvers during a normal ILS approach.
Cheers...FD...
