The higher pitched noise from rally cars is because of the huge boost that they run and the short gear change and the fact that there flat out when changing gear

You were mearly driving so your car doesnt create as much boost, hence why it was a slow dump valve`ish sound if you like

The chatter is the air trying to return through the turbo when you lift of the throttle and the blades chop up the air, the faster the turbo is spinning the smaller the "chops" are hence the higher pitched noise

If anyone wants to correct or tidy up my description feel free