depends on the turbo too.........My girlfriends saff used to chatter like fook on about 23psi with a T34 and a cone filter but my Escos runs 2.1bar (31psi) on a GT28 roller bearing turbo and with the cone filter fitted, it only just say "flutters" ............thats why I took off the filter and put the airbox back on as it is quicker and more responsive with the box on.

It isnt actually the wastgate chattering, its the boost pressure that hits the compressor wheel on vacuum when you back off........so I've been told. thats why you hear it through your filter