Garage plus two and a half week old ducklings means I would put my nose at brooder level before anything else. They soak bedding astonishingly fast, wet bedding makes ammonia, and ammonia is a genuine airway irritant that reads as sneezing before it reads as anything.
Change it more often than feels reasonable, use large flake pine rather than cedar, and get air moving without putting a draught on them. If it smells sharp when you lean in, that is your answer and it is not the feed.