CPAP, or continuous positive airway pressure, is the gold standard treatment for obstructive sleep apnea. Quite a few advances have been made in CPAP in recent years, including noise reduction. As of 2010, CPAP machines should generate a similar noise level to a fan in the room. This also applies to the CPAP hose. If your hose makes any kind of whistling sound, investigating for leaks and adjustments usually fixes the problem to facilitate proper treatment.
Remove the CPAP mask from the hose by gently twisting and pulling. With your CPAP machine connected to the hose, plug the loose end of the hose and turn the machine to the "On" position. If this eliminates the CPAP whistling, the sound is likely coming from the CPAP mask rather than the hose. In this case, you'll need to have your mask refitted to solve the problem.
Make sure the CPAP hose is securely attached to the CPAP machine by pushing and twisting the rubber end all the way to the surface of the machine. If the hose is not all the way connected to the machine, air will leak and whistling can occur. If whistling persists, your hose may have stretched or loosened so that it no longer makes a secure connection with the machine. In this case, you'll need to replace your CPAP hose.
Fill a bucket with water. Remove the hose from the CPAP machine. Submerge the hose in the water and allow it to fill. Plug both ends of the hoses with your fingers and lift out of the water. Observe for any water leakage from the hose. If you notice leakage, a hole is present. If so, you need to replace your hose.
Consider how long you have had your CPAP hose. In many cases, whistling results from a worn out tube. Based on information from CPAP.com, "signs of wear are dry, cracked places on the inside lining or on the rubber ends; stretch marks near the rubber ends; and mineral deposits or mold from water left inside the hose." If you see any of these signs, you should replace your CPAP machine. If it has been over a year since you replaced your hose, replace it, since a well-worn hose is a very common cause of hose whistling.