Airliner - "barking dog" after take-off and before landing I've always wondered what is that noise. It sounds like a barking dog or an air drill. 
Sometimes you hear 4-5 "shots" but last time I flew the noise went on from before take-off to long after we were in the air. 

It looks like they are tightening something. Am I correct?
This sounds like the PTU, used to pressurize the hydraulic system when only one engine is running. 

http://www.askthepilot.com/questionanswers/the-barking-dog-airbus-noises/