this is a quote from http://en.wikipedia.org/wiki/Wastegate
Wastegate chatter myth
The noise is in fact the air compressed by the turbo passing back through the compressor housing of the turbo after having hit the now closed throttle plate.
A blow-off valve (BOV) will usually eliminate this noise, but in some cases, i.e. where it doesn't open fast enough or is setup to only react to high boost, some chatter will remain.