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
Engine miss.
There could be many reasons, a vacuum leak is one possibility.
When a vehicle is idling, the engine may not produce enough heat to warm the coolant effectively, leading to cold air from the heater. In contrast, while driving, the engine works harder and generates more heat, allowing the heater to blow warm air. The engine temperature can rise while idling due to reduced airflow through the radiator and less coolant circulation, while driving increases airflow and coolant flow, helping to cool the engine.
A miss or A collapsed engine mount or A imbalance caused by a failing component, harmonic balancers and torque converters are common.
air leek
It is the idling sensor on top of the intake
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