2001 Mercury Sable Very Fast Idle

My daughter's car is idling very fast. I realize it is winter but it is very fast even though. She recently had the car break down on a trip to the tune of approximately $1300.00. The following work was done: Replaced the AC Compressor package and recharge. The vehicle was making a grinding noise apparently coming from under the hood. The noise disappeared when the vehicle was put in neutral. The vehicle worked fine on the return trip after the repairs. The vehicle then sat for a couple weeks because she was sick and because she has a crack in the windshield (whole other story). She is currently unemployed so my question is this: What simple inexpensive things can we check before taking it to a garage? She really can't afford to pay to have it checked and repaired and then find out it is something simple she could have checked/fixed herself. Also I have some experience and feel confident, depending, working on cars. Thanks