Damp smell from fans when not using air-con

[Hequn]

Hi guys, just a quick one. When I turn the fans on in the car I get a damp smell come through but if I use air con it goes away. If I turn the air con off and use the normal fans again, back to damp.

Does the system need some form of clean? If so what's the best way?

 

[Hequn]

Cheers dude