Fan Noise: Excessive fan speed is the biggest noise source in this area and can be reduced by using a temperature controlled or thermostatic fan. 

Other noise concerns are bent fan blades and broken or missing shrouds.

Air Intake System: Both the type of system and the location of the inlet affect the truck noise level. Air inlets that open to the side are generally noisier than those that do not. Intake air silencers are available for problem applications.

Mechanical Noise: - The engine and drive train are the major source of noise. 

Operating speed, type of engine and drive train all affect the total noise level. Generally, any part of the engine or drive train that you can see while standing away from the truck will generate noise.

Shielding the exposed areas with acoustic barriers will reduce noise levels. 

Tires: Many times this is the major noise source at high speeds. 

The condition of the tires and the tread pattern affect the noise level. At low speeds this is not a large source of noise.

Exhaust System: Worn or inadequate exhaust systems are the largest and the most frequent contributor to high noise levels.

A visual inspection and repair of leaking connections or holes in the system components will noticeably reduce noise levels. If the noise level is still too high, the addition of resonators, packed stacks or a change in mufflers is recommended.