Star Citizen's recent ship production pipeline video had an overview of their thruster audio. It's not a perfect reference since they're not trying to reproduce a real-world sound, but it might be of interest: https://youtu.be/Bbq2Crt8pTA?t=1525

Segment on engines starts at 26:44.

EDIT: for anyone who can't/won't watch the video, I think it's a bunch of synth and sample layers that shift at different rates as the thrust changes. He only specifically talks about a few synth layers though.