HEARD A POP SOUND FORD FOCUS 2004?

My cousin was driving my car it is an automatic ford focus 2004 the car has no problems what so ever however today when he was driving because he is a beginner he had hes foot on the brake all the time while steerinf and changing gears and he was changing from d to reverse and i heard abig pop sound its driving fine still gears still changinf quick and and no apparent problem however i'm worried as to what it was can anyone help? And is it something for me to worry about i'm scared something bust in the car

UPDATE as a description of what happened as soon as he changed the gear from D to R a loud pop sound came from the outsidelike it came from maybe the gears?

If all you can say is big pop there's not much hope you will get anything useful here.

H e most probably ran over an empty carton or bag in the road.