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SYD-LAX is always a LR, so I'm guessing the OP heard this on a LR.
On Airbus narrowbodies (A319/A320) the "barking dog" sound you hear is the hydraulic power transfer unit (PTU) turning on and off very quickly; if you google "A320 PTU airliners.net" (without quotes) you'll find a bunch of threads on that site (usually after the 3rd or 4th reply) that provide a lot more insight as to what's going on.
-J.