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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?

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    $\begingroup$ I hear a "thump" at about T-3 but not a Titan-like "whoop". I don't know what T+0 is keyed to in the Atlas launch sequence but according to spaceflight101.com/atlas-v-tdrs-m/flight-profile ignition occurs at T-2.7 seconds. $\endgroup$ – Russell Borogove Aug 25 '17 at 4:07
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    $\begingroup$ Rather than something specifically associated with the starter cartridge, I suspect the Titan's "whoop" is the sound of the turbopump itself coming up to speed, but I've got no hard evidence to back that up. $\endgroup$ – Russell Borogove Aug 25 '17 at 4:18
  • $\begingroup$ @RussellBorogove The sound of Titan may be due to the flow of the propellants from the starting cartridge through the inlet nozzle of the turbine, as this article suggests. I am not sure about the credibility of this. Here is a video of a Titan II launch. $\endgroup$ – nish-ant Aug 25 '17 at 6:41
  • $\begingroup$ @RussellBorogove T+0 for Atlas V marks the booster ignition. This means that the source of sound is not the exhaust. It has to do with the engine-startup at T-2.7s as you suggested. EDIT: Link added. $\endgroup$ – nish-ant Aug 25 '17 at 6:45
  • $\begingroup$ For the Titan II, see the explanation on Vintage Space youtu.be/WTJDI4bwtOM more details here: popsci.com/why-titan-rockets-launch-bwoop#page-3 $\endgroup$ – oefe Sep 2 '17 at 19:42
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If it's a rocket launch, happens at around engine ignition, and it sounds like some variation of 'whooop', you can bet it's the sound of turbomachinery very rapidly spooling up. Consider those turbopumps go from zero to tens of thousands of RPMs in fractions of a second, they cover the whole range of audible frecuencies, fast.

I actually ended here while looking for a good clip of Soyuz's awesome sound during startup, just because it is an awesome sound, but I am having rouble finding a good one. Whoop!... and then the roar of the engine at full power.

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