I changed the pollen filter and it made a small difference to the noise. The good news is that I discovered, through trial and error, that the noise level is dependant on where the direction of the air flow is set. Thus when it is set to the 12 o'clock position i.e. pointing to the windscreen, there is no noise - however turn the dial to the 1 o'clock position and the noise starts. As this is the only position that I can hear the nosie, I am quite happy to avoid this setting. However, the noise does disappear when the engine heats up.