2005 Ford KA air making a loud buzzing sound

Hello People I could do with some help! I’m giving girls a bad name here but I admit I do not know the first thing about cars and I have a problem which I can’t fix!!! Ok the problem, I have a Ford Ka and the problem is to do with when I put the air on (not air conditioning; it’s the standard air blower). When I crank the air on to 3 or 4 setting, hot or cold, there is an awful loud buzzing or wurring noise and it seems to be coming from the passenger side of the dashboard. I’ve only noticed this noise twice when it’s been raining when I’ve needed to de-steam my car but obviously the problem isn’t going away. Can anyone help please?