Clucking from defrost

[Trouble maker]

Just bought a 08 f250 and noticed when I have the defrost on ever 3 mins or so you hear a clunk (almost sounds like old school defrost vents opening/closing). Now it only does this on defrost mods. Anyone know what it is?

[ryandamo]

mine does that too, i posted about it before but forgot what the answer was, im pretty sure it involved taking the dash out so i left it alone