I recently travelled with an A320 and an A321 from FRA (Frankfurt) to TXL (Berlin) and back again. On this and on previous flights I already noticed a whining sound, much louder than other sounds, while we were still at the gate or soon after leaving the gate and starting taxiing. I cannot remember exactly. My research showed that this was the PTU's sound (aka “barking dog”), typical for an A320 and its sisters.
One of my references was this post. The author said:
[The PTU can be heard]
At the gate, especially if we taxi in on one engine. During the taxi, the left engine is running and pressurizing the green system.
This led me to this question:
When taxiing with one only engine, do pilots always use the left-most one (#1)?
Or do they randomly choose one of the two (or four)? If they always use the port/port outer engine, doesn't it wear out sooner? Or are the engines swapped at regular intervals (like car tyres are swapped from front to rear)?