Some sprited driving and out of order transmission

[autoque]

'08 X5 3.0si with 8400 miles on odometer.

I was driving on a stretch of highway at a high speed for some 30 miles using more kickdowns than usual(~10 to15 times) and staying more time in between 4000~7000RPM (more than usual anyway). Then off the highway and I was going at around 35mph. When I stepped on the accelerator to merge, but not using kickdown, I felt some change in gear selection then bong~ sound came and a messege about an out of order transmission came up saying to drive slowly.

When I got home driving the vehicle at around 37mph, I could smell something burning from the front kidney grill area, called the service center and came out to drive it to the dealer. I started the car but the message disappeared and everything seemed back to normal. I took a quick picture of the message just in case this happened, though the error is probably recorded in the vehicle's memory.

It's in the shop but any idea what it might be? I don't usually drive like I stole it.