Definitely engine noise, and definitely worse the further forward you sit. it's very noticeable with the CFM engines on NW (seat 1A), and US (seat 2A), but not the V2500's on UA (seat 1A) or BD's 321 (don't recall seat).

i got stuck in 4A on a NW 319 a couple of weeks ago, and even there, it's not nearly as noticeable. but i usually sit in 1A, and i usually hear the buzz very well.

i have a memory of floating in the pool at the wyndham in miami, and hearing an aircraft buzzing by overhead. i thought "that's gotta be a 10." but no, i looked up, and it was a NW 320.

--Dan