Sounds to me like excess drippings hitting the coals and vaporizing. That happes a lot with large cuts or fatty cuts like chicken or bacon. If cooking more than a little amount of the latter, things can get nasty pretty quick when the fat starts rendering out. The fix for this is a second grate with a diffuser or better yet a drip pan with water or sand in it.
Could be bad airflow though as others suggested. Question- I read they're website and they listed a "fire box liner". What is that exactly? Being that it's a drum and there's no firebox per se, just curious if this liner is restricting your bottom intakes in some way.
UDS'55&16, Akorn, Bar-B-Q Kettle, mini WSM, Drum Grill. I'm JD.