Pinking is predetonation of the firing cycle, i.e. the fuel in the combustion chamber is burning too early. in older cars it was due to the timing being to far advanced or the incorrect grade of fuel being used (to lower grade) or even a build up of carbon mainly noticeable when under load, but with modern engines they have anti-knock sensors which can alter the timing to suit the fuel being used, pinking sounds very much like a length of chain being rattled, it can lead to pistons being holed, been there, done it, repaired it! the old Rover 2000 tc needed 5 star fuel, giving my age away lol!