Why does my 2000 Hyundai tiburon manual trans rpm's go up to 2000 when taken out of gear and does not go down until completely stopped?

I have a 2001 Tiburon with the DOHC 2.0 and had the exact same problem. Turned out that the intake gasket was leaking. I found this out by spraying brake cleaner around where the intake bolts to the head on the backside of the engine. When you spray it more than likely it will burn white smoke out of the exhaust and possibly change the way it runs. I change the gasket myself but I'm also a mechanic so if your not comfortable attempting to replace the gasket I would take it to a shop.