Ambulance
Bear
Circle
Cliff
Fall
Fire
Flute
Horn
Lap
Millionaire
Mug
Parachute
Plane
Scientist
Suit
Train
Unicorn
Plane seems obvious, and then parachute.
After that, I see a couple choices:
Circle - the classic move of an airplane dogfight, and also dog fights happen in rings
Suit - pilots wear suits
Fall
Ambulance - siren, not whistle, but worth considering
Circle - nah
Cliff - nope
Fall - no
Flute - best match for whistle so far
Horn - another instrument, but not as good as flute
Lap - no
Mug - no
Scientist - no
Suit - no
Train - yes
Unicorn - no
if it were flute+horn then makeup train then he woulda done instruments - 2
I think train/flute is our best option for whistle - 2 and then that would leave unicorn for our final word
flute+unicorn seems hard to clue for; if horn wasn't on the board, he coulda done horn - 2 as unicorns have a horn and horn is an instrument like a flute, but unfortunately horn is on the board...I can't think of another clue for flute+unicorn, can you?