Originally Posted by Thrush

Chatter occurs when escess, built up charge gases get stuck between the inlet butterfly and the turbo when you snap the throttle shut when changing gear of lifting off for example.

If you have a dump valve (either to atmosphere or a recirc type that vents backinto the airbox) these gases are dispelled either to atmosphere or re-circulated back to the airbox.

If you have neither, the gases are stuck, If there is enough force and enough gas, they hit the now blocked off throttle butterfy, turn round and go the opposite way down the charge carrier. They exit through the turbocharger and the noise you hear, the "chatter" is the sound of the excess charge air being "chopped" by the turbine blades.

So, you will get more chatter if you have no dumpvalve at all. There is an argument as to at what point you NEED a dumpvalve due to the mass airflow going backwards through the turbine blades causing turbo-stall and possibly damaging the bearings and shaft.