2001 Kia Sephia

My husband was having issues with the car going into reverse and drive and the other day I was trying to leave and the car hesitated to go into reverse and then into drive. It finally seemed to go into the right gear about half way down the road. When getting on the highway the speedometer said 60 but I felt that I was only going 30 then there was a burning smell. The car is also now making a very loud rattling/knocking noise also when this was all starting to occur.

What do you think, could it be the transmission or is it something else?