For reference, you can hear the distinct 'whoop' sound at T minus 3 seconds mark of the recent TDRS-M launch. This sound has been distinctive for most previous Atlas-V launches as far as I can remember.
I have came across a similar observation during the Apollo-era Titan II launches. The reason for the pre-liftoff sound here is the flow of hot combustion products from a starter cartridge into turbine inlet nozzle under high pressure. The turbine drove the turbopump to supply the propellants to the combustion chamber.
I assume that the sound in Atlas-V has also to do with the engine startup. I found a explanation of the startup method of the Atlas-V RD-180 engine (Russian article here). It also involves a startup cartridge which contains the staring fuel. During startup, the cartridge breaks under the action of pressurized fuel released by a reservoir and the fuel contained in the cartridge flows to a gas generator. I am not sure if this pressurised flow of the starting fuel is reason behind the startup sound. If yes, what is the exactly causing the sound? If no, what may be the other sources of the startup sound?