chatter??

[Ruan]

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.

Wouldn't cause a chattering noise, all you'd hear is a low cyclic noise from the exhaust of the wastegate opening - I've had this before, it happens if the line from the boost controller is too long, it takes a while for the lines to expand and contract and the pressure to pulse up and down the pipe, causing it to over/underboost... My VNT does kinda the same thing with the vanes, when you stamp your foot in it, it reacts SO fast, it'll just overboost, then open suddenly, then close... Doesn't cause a chattering noise though...

Compressor surge is simply the airflow from the compressor not being stable due to it being on or over it's surge line - Surge line being the compressor is putting out too much pressure vs the shaft speed as the airflow demands aren't there, so the airflow isn't stable... I.e. if you're ON the throttle and it's happening, the turbo is being restricted on it's inlet to the engine or on it's way out - the exhaust (If you want to hear ON throttle surge, come in my car, you'll see what I mean, it's a horrible hissing clicking noise from the compressor, keep going, you'll get the typical "bark" from the compressor)...

OR you've just lifted off and the revs are falling, causing the airflow demand to reduce massively, pushing the compressor right into it's surge area as it spools down, causing that WHOP WHOP WHOP noise from the compressor as it slows with the airflow very unstable.

No means to blow my own trumpet, this is an area I'd say I've got a good bit of knowledge...