The short and only right answer is: none. Any sound made by the turbocharger can be considered a nuisance by anyone inside or even outside of the vehicle. During the development of a new turbocharger, a lot of tests are performed to check if the turbocharger is making any type of sound. The most infamous sound which can be made by a turbocharger is the whistle caused by the unbalance of the turbocharger rotor. There are also other sources of sound which can originate from the turbocharger: from the bearing system, the compressor wheel, the wastegate and from the bypass valve.
What do you do to make sure that my turbocharger does not make any sound?
Many of the sounds described above can be prevented by making a good turbocharger design. Many of the quality improvement features in the turbocharger can’t even be seen by the human eye, but determine whether you can hear the turbocharger or not. Particularly the compressor has to be carefully designed so that it delivers the necessary boost at an acceptably low sound pressure level. Another part which may cause noise is the waste gate, which can cause a rattling kind of noise when improperly designed.
Also the balancing quality of the turbocharger strongly affects its noise level. During the production of a turbocharger, a lot of effort is put in minimizing rotor unbalance. First, the compressor wheel and turbine wheel are separately balanced to meet the strict unbalance requirements we desire. After assembly of the cartridge (the core of the turbocharger containing the rotating parts), the complete rotor is balanced again to meet the requirements. The turbocharger rotor is produced in such a way that the rotor remains well balanced during its complete lifetime, and not just during few kilometers of use like many low-cost imitation turbochargers.
The bearings which are used in Mitsubishi turbochargers are especially designed to minimize vibrations and prevent the occurrence of bearing noise (the so-called constant tone noise). As the turbocharger interacts with the combustion engine, the sound can be heavily influenced by other parts of the vehicle, so close cooperation with the vehicle manufacturer is required to prevent turbocharger noise under any operating condition in any vehicle.
So if my turbocharger makes strange sounds, it means the turbocharger supplier has done a bad job?
No, the turbocharger will only go into production when the turbocharger is fully validated and the vehicle manufacturer is satisfied with the results. Although the turbocharger looks like an invulnerable piece of metal from the outside, it is actually an advanced piece of technology, operating at extremely high temperatures and rotation speeds. To achieve these high speeds, the rotor is supported by bearings which rely on a very thin film of oil to function properly. If these bearings are lubricated by filthy oil, they will eventually wear out and lose their functionality. This may lead to a noisy turbocharger. Also particles which enter the compressor due to a malfunctioning air filter can do serious damage to the turbocharger. Therefore, it is key to strictly follow the service intervals stated by the vehicle manufacturer and use the correct oils and filters. This makes sure that your turbocharger delivers its power in silence for many, many years.