Might be worth checking the hot bleed line to the mixer box for the cabin heater/demister (under the P2 seat) never good to have a bleed leaking.
Check for bits of paper or leafs that might have got stuck in the de-mister vents.
I once had a post-it note doing a great harmonica impersination until I tracked it down to a vent (kids love to stick wierd things in odd places)
Does the whistle only start when you are in flight idle or is there on ground idle too?