I think baby can begin to hear around week 20 or so. However their ears could be developing and I wouldn't want to risk anything. Most doctors or midwives tell women to take a pillow and hug it against their body tightly to help further muffle noises to baby's sensitive ears.
However I also have to point out that while in your womb your heart beating is very, very loud to baby. I would look up the decibels of the Indy compared to decibels in the womb to an unborn baby for a better understanding of how safe or unsafe it might be.
Lots of people go to concerts or events while pregnant without a problem; I've seen a lot of pregnant women do the pillow thing, LOL! They look and feel silly but in the end they have a good time and feel better about doing something to protect baby's ears (if anything; again not sure on the decibels!).
I would talk to your doctor. I know I personally went to the NHRA drag races when I was 28-29 weeks pregnant. There were a few other pregnant ladies there much further along than I was as well. It was super load, was constantly shaking the stands and underneath them, and super hot. My midwife just told me to avoid the exhaust fumes if possible. (meaning don't stand behind the cars when they started them) With my daughter, the only thing the sound did was make her move a little more and jump a few times. I honestly doubt (strongly) you would have any problems going.
That might be something to ask your doctor. I know later on your pregnancy the development of his ears is a crutual point in his growth and he can hear everything except it sounds as if he were in a bowl of water. I dont think it would hurt him at all but then again i have never been to something like the indy 500. I wuld suggest you call your doc on that one! Good luck and have fun!