Had this problem on my 4 after I had removed and reinstalled heater,, fan would make a noise not unlike clip, when i took front cover back off of heater (fan housing) I could see slight friction marks on the circular part of matrix housing pictured above,the fan itself is mounted by 3 bolts, behind casing, inbetween motor and casing, on mine these 3 fixing bolts had 4 shim washers, so the axis of fan could be slightly adjusted by the movement/addition of these washers, I looked at friction mark on casing ( if its making a noise, it will be leaving a mark).and moved shim washers so axis would change stopping fan itself coming into contact with casing.
Bit involved and trial and error ( assembled once, checked , dissasembled, adjusted, reasembled) but effective in getting the fan to run without undue noises.
Also noticed that the matrix housing fixing to the bulkhead itself needed tightening the same amount each fixing, if tightened unevenly it throws the shape slightly out making fan contact more of a possibility.