Noisome does not mean loud at all; it means offensive or harmful, especially an unpleasant, often foul-smelling, quality, as in a noisome smell from the drains. Noisy is the word for loud or full of sound, like a noisy engine. The similarity in spelling is a coincidence, so do not read noisome as a fancier version of noisy.
The core difference
Noisome means offensive or harmful, often to the sense of smell, while noisy simply means loud.
- noisome — offensive or harmful, especially unpleasant-smelling: a noisome odour rose from the drain.
- noisy — making a loud or unpleasant sound: the noisy engine made it hard to hold a conversation in the car.
How to tell them apart
Noisome comes from an old root related to annoy, built from noy plus the suffix -some; it has nothing to do with the word noise. Noisy, by contrast, is formed directly from noise plus the adjective ending -y, so it is a much more recent and much more literal formation.
A useful trick: noisome pairs naturally with smell, fumes, or vapours, not with volume, so if you can replace the word with foul or harmful and the sentence still makes sense, noisome is correct. The mix-up shows up most often in older or more formal writing, where a reader assumes noisome describes a racket rather than a stench.
