I am surprised your flight instructors have not told you what it is. Its the stall horn. Landings are to be done, most of them anyways, to a FULL STALL. If the stall horn does not go off, you did it wrong. On newer models, the flow over the wing changes, which creates a suction. This in return makes the stall horn go. In older models with an electronic stall horn, its not a vent in the wing like on the newer models, its a little switch. When the flow over the wing changes like in a stall of moments leading up to it, it will throw the switch up and connect the circut, allwing for the horn to sound. The stall horn will go off from stall to 10 knots above stall I believe. If I am incorrect, I am sure one of the CFI's here could explain this in MUCH better detail than I can.
23 victor, turn right heading 210, maintain 3000 till established, cleared ILS runwy 24.