Just coming up to 2 years of C4 ownership. Was going to report and all's well - well more or less - then a weird whooshing noise in the cabin starts on Friday as you touch the accelerator. It dies away after about 1.5 seconds at the same revs. Louder if you floor it but still dies away. Seems to be coming from ventilation system maybe the windscreen vents. No setting of the ventilation system makes any difference that I can tell ie. fan off, a/c off, screen demist on/off, fan max, temperature, auto anything. Doesn't do it when engine isn't under load ie. rev with clutch disengaged. Any ideas anyone ?