The air box is partly responsible for tuning the engine sound, but more than likely at the specific rpm the engine was vibrating just right to vibrate the air box against metal, that would be my guess. More or less engine load would change quite a bit, as the engine rotates slightly within it's mounts as the torsional load is applied. Find just the right angle, and it moves the air box to the perfect spot an voila you've got a vibration.
Granted this is just a theory based on the information present, I've seen similar issues with exhaust vibrations caused by a worn motor mount.
2019 Insight EX, Aegean Blue (Owned by my someday wife)
General Automotive knowledge, Degree in Autobody, worked in Electronics for the past 10 years.
Retired Professional Car Audio Competitor. Avid knowledge buff, always researching something.