Driving home this evening, there was a car a bit behind me, weaving in and out of traffic.  I'm in the left-hand lane on a two-lane highway and eventually he pulls in right behind me, so close that I can't see his headlights.  It's heavy traffic and the person in front of slows down, so I have to step on my brakes to slow down as well.  

Knowing this car is right behind me, I glance in my rear view mirror and see him pull out into the right-hand lane and accelerates.  As soon as he does, I hear two very loud pops a half-second apart from where this car was.  He takes off past me and continues weaving in and out of traffic.

Now normally I wouldn't make anything of hearing a pop or two, but because of the erratic driving, the volume of the pops and the circumstance of when it happened, I'm considering that I was shot at.  When he passed, I saw that he had his window down, so it wouldn't be out of the question.  I didn't see any weapon.

So just in case I need it, I catch up with this guy (doing 90+) once traffic clears up so I can get his license number.  I saw part of it when his light blue Saturn went by, so I know I had the right car.  When I got home, I checked the back end of my truck and didn't see any bullet holes.

Maybe I'm just being paranoid, but my gut tells me those weren't backfires, which is why I post this question.  How common would it be for a Saturn to backfire, under what circumstances and how loud would it be?