Could be lots of things, I would start by checking fresh air system
open close face vents see if noise changes or goes
Change the direction of air flow to footwell
Close off all vents and fans etc etc
Might be a water valve try turning heater on and off
Could be an electrical relay
Just keep on trying different things and see if the noise changes which will give you an idea where to start looking further.
Maurice, August 2010
