Yes and no. In general, they're much quieter than your average fish tank or central heating unit. However, the noise level can vary depending on the setting on the oxygen concentrator you're using, how fast it's running, and how much outside sound is reaching in through its casing. With all that said, an noisy oxygen concentrator is still better than not breathing because of momentary annoyance with a sound. As long as it does what you need it to do efficiently and easily enough to manage without any issues--then who are we to judge?
There are two different types of oxygen
concentrators. The Jet O2 Concentrator is the most popular one, and it is as loud as a vacuum cleaner. But they have developed quieter machines that are smaller in size called Pulse CVT and Pulsair.