I have a 2002 ford focus. The past week when I put my car in park to idle, lets say at an atm or to turn off the car, it starts revving up. I made sure I wasn’t touching the accelerator. I made an appointment to get it looked at, but is this serious. It doesn’t happen every time but at least once every 2 days. thank you for any advice 
What happens when the car is still in gear but you are sitting still - such as when you leave the car in drive while sitting at a traffic light or something? Is it generally idling higher than normal? Does the idle surge and fall at all? Is the check engine light on?
I think that the first likely culprits would be a sticking idle air control valve, a dirty throttle body, or a vacuum leak. Clarification on the above might help though.
I’m going to suggest the fault MAY be due to a loose or broken throttle position sensor (TPS).
A broken or loose TPS can cause intermittent bursts of fuel from the injectors and a unstable idle because the PCM ‘thinks’ the throttle is moving.
When the car is in drive and I am idling it sounds normal, it only happens when I put the car in park that it starts racing. No check engine light either which is why I am confused.