One synonym for the phrase "bug in one ear" could be an obsession or fixation. This is when a person becomes overly interested or preoccupied with a specific idea or thought. Another synonym could be a nagging thought or persistent worry. In this case, the thought or concern keeps recurring, even though the person may try to push it away. A third synonym could be a mental itch. This refers to a sensory experience where a person feels as though something is crawling or tickling inside their mind. These three synonyms capture the essence of the phrase "bug in one ear," which describes an unwelcome or distracting thought or sensation.