Could be something getting loose under the car, maybe a screw fell out and a cover a vibrating a little. Happened to me once.
I could have sworn BMW needs 91, but I'm no expert on what low octane can do except for reduced performance.
But definitely try to find an external source for the sound, and narrow down where the vibration is coming from.
2010 E93 M3 / AA Catless X-Pipe / AA Catback / AA Pulley / Vorsteiner Vented Hood and Bootlid E93 Trunk / ADV1 7.1 Wheels GB / LS3 LED Lights / AFE Stage 2 / Velos Designwerks Tune