Pinging?

I have a 1989 2+0 TT Manual,
i took my car for a full proper service. They replaced my engine oil, gearbox oil, diff oil, radiator flush etc and changed spark plugs to NGK standard.
After this service, whenever im in boost at high RPM the car starts pinging.

Does anyone know how or why this has happend? and how to go about fixing it?
Cheers

 

boost leaks, They havent sealed the balance tube back up properly after pulling it to change the rear plugs. Bet they left the Orings out.

 

That sounds about right, thanks for your help... ill have to take it back to the shop

 

Pinging can also be cause by incorrect ignition timing or too low fuel octane rating.