I also remember someone a long time back explaining that if the metering head was fooked then that would also contribute to the effect of chatter.

Obviously it's the air stalling back through the turbo making the noise, but perhaps if the air-flap remains open (due to being fooked), that creates low pressure air that side of the turbo, then the higher pressure air from the other side wants to get to the low pressure side (due to normal air physics of low pressure/high pressure areas), so moves back through the turbo at a quicker velocity, causing a louder chatter noise than if the metering head was closed fully and the pressure on that side was slightly higher with a lower velocity movement result.

James: what difference does having a 0.48 housing have over standard ?
Previous owner of my car was told by Karl at NMS that the turbo had a 0.48 housing.