It is the idling sensor on top of the intake
air leek
get an engine diagnostic check, a lot of different problems can contribute to your situation.
If you mean why the difference in RPMs between idling and driving, the difference is due to the engine load; At idle, the engine isn't really doing much other than staying running, but under load (driving) the engine has to put its power out to the drivetrain, hence the symptomatic loss of engine speed (RPMs).
Your thermostat is stuck open. Buy a new one they are usually really cheap and you can usually repair it yourself. They are easy to get to even in compact engine space.
A car that is stopped with the engine running.
Yupp Clogged fuel filter. Hole in the gas line.
In order to tell you that you would have to describe the noise and what the car is doing when the noise is made. Are you idling, driving, just turned the AC on? What is occurring and what is the noise? Idling and sounds worse when AC is on. Sounds like it is coming from the top of the engine. Vehicle has 120k miles
An engine revving to high RPMs could be a symptom of many conditions. The potential cause?æalso depends on whether this happens?æwhile idling or while driving.
water in the fuel can cause an idling motor to sputter and/or die.
As the vehicle sits idling the driveshaft is not rotating.
It could be a variety of things, all having to do with the cooling system. Most likely it is a faulty engine fan that is supposed to automatically cut on when your engine hits a certain temp. It could the the fan motor itself, the relay that controls the fan, and maybe just a fuse. If the car does not overheat AT ALL in freeway driving (lots of cool air passing over the engine constantly) and overheats either idling or during city driving that engine fan is to blame.
asuming its a blazer around the majority of the others on this site a pluged fuel injector or a spark plug wire is rubbed and touching a ground point