chatter??

[Dum-Dum]

Right basically its at part throttle near the boost threshold of the wastegate (ill use 15psi as an example) and often if a wastegate is too big for the application.

So say your crusing along, relatively quickly part throttle just hitting 15psi. Wastegate opens, lets out too much gas, pressure drops, wastegate closes again, takes a split second to hit 15psi again, re opens, pressure drops again, gate closes and so on and so on untill the throttle position / revs change.

Its that situation that does cause wastegate chatter but it sounds nothing like people think it does and I strongly doubt a car on its standard turbo, especially a diesel would manage it.