bearing
radiator fans running to cool engine...
Bumps in the road.
If clicking while trying to start then might be a bad starter. If clicking while running, then might be valve/lifter noise.
The fuel in the cylinders exploding
you have a control vacuum leak, listen for hissing noise under dash when engine is running.
when the engine weres down
The fuel in the cylinders exploding
No, not really. An engine will run without an exhaust (but you may get arrested). The exhaust simply directs waste gasses and quietens the noise. It plays no part in the running of the engine, but can reduce the performance if it causes on obstruction to the gasses.
If the engine running you hear the engine rev up. Describe noise, is it when in park or when driving? Does it hurt the performance of the vehicle?
lifters
A bad compressor belt can cause a whistling noise while running the air conditioner. A bad engine can also cause this noise.
Most of the time ,it's because engine is catching air .check for a hissing noise. a soapy solution in a spray bottle .while engine is running will find where the leak is .engine sound will change when you find the spot .