Pros use DSLR cameras, and when the mirror slaps down so the sensor can be exposed, it makes a noise. There are other boops and beeps that can be turned off, but the mirror mechanism is going to still make noise.
There are now mirrorless dslr. No need for slapping mirrors. My 5 year old samsung nx1 has silent sbooting and high resolution. Can take 4k video too. Silently.