I think it’s on the trailer brakes but why do the air brakes on some lorrys play a little choon when you turn the ignition on 
It’ll be the trailer EBS/ABS system performing a self test. Hence it firing of the air valves.
Ah ok, takes little to amuse me
I am amused by this solo as well 
Mine also flashes the headlights when I fire her up (well switch ignition to pre engine turnover setting thingy).
I also had a trailer the other day that blew off little musical fanny ■■■■■ of air if stationery for to long, makes it sound like im getting impatient and ■■■■■■■ about with the brakes at junctions. [Edit: Missed opportunity to insert ‘■■■■ and give us a clue’ joke]
Maybe a fault or oversensitive valve or something else technical but it was Friday so aint bothered. 