It's the 14 stage bleed valve. It bleeds air off of the high pressure compressor. The exhaust for it is on the left core cowl of the engine so the bleed from right engine will be louder in the cabin than that from the left engine but both are clearly heard from inside the cabin. Some have suggested it is the 2.5 bleed (fan air bleed) but the pressure from that is relatively low and it dumps into the bypass duct so the noise from it is muffled and not easily detected from inside the cabin. The 14th stage bleed valve opens to unload the compressor during start and shutdown, and provide compressor stall protection at certain low power setting conditions, engine transient conditions(acceleration-deceleration), and reverse thrust.
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