The unit could be emitting an odor for a variety of reasons.
The carbon filters are required to ensure a near-odorless cycle. They last approximately 500 cycle hours, or 3-4 months with regular use.
Even if the ‘replace filter’ light has not gone on yet, you may need to switch out your filter carbon. Similarly, you can continue cycling even if the filter light has gone on and no odours are apparent.
Filter life is determined by the moisture quantity in the food waste, and the natural odorosness of each cycle. If you are cycling rotten, smelly or potent food wastes (such as rotten meat, or old cheese) your filters may not last as long as if you were cycling simple veggie scraps and coffee grounds.