In a composition that I am writing, I am describing the sound of a very large hovercraft, namely its propellers. I've listened to a video of one such hovercraft, and it's not a whir, or a buzz, or a drone. I just can't quite put my finger on what to call it. The best I thought of was "roar", but I think there's probably a better word than that.
I suggest you have a look at the video (Skip to about 15 seconds in) and see if you can think of a word to describe the sound.
Sample sentence: "I yelled under the ______ of the three vast propellers."