Popping/Backfiring

I have a 2008 Prius. I know various noises have been discussed on this forum but I haven't found an answer for the one I've been experiencing.

I have occasionally heard a popping or backfire towards the rear of the car. It seemed to be more present on my last trip where I was climbing an incline and there was a altitude change. I did accidentally put 85 gasoline in the tank once but that has been run through.

Any ideas?