There are several factors that can contribute to the sound level of your gun. If you think your gun is too loud please consider the following...
Are you shooting a pellet or dry firing?
This makes for a far louder shot. Make sure a pellet is properly loaded for every shot.
Some guns such as spring and gas piston air guns can be damaged if dry-fired. DO NOT shoot these guns without a pellet.
Are you using a heavy enough pellet?
Some guns can shoot pellets faster than the speed of sound. This sonic crack cannot be silenced by any suppressor or baffle. To reduce this, shoot a pellet at a velocity of around 1000 fps or less. This will keep your shots accurate and quiet.
Are you indoors or outdoors?
Depending on where you are shooting can also change how loud an air gun sounds when it goes off. Closed-off spaces like a basement can echo the report of an air gun.
If the gun is suppressed, are the baffles intact?
Many guns come with baffles and suppressors on them from the manufacturer. It is possible that they are damaged or missing on some guns which can change the report. Make sure to visually check for damage or missing pieces.
Is the gun just plain loud?
Some guns are loud. They either have no suppression or the power plant itself such as CO² has a louder report. In these cases, there may or may not be anything you can do to make them quieter.