It's crud in the air valve. It happens from time to time. Not difficult to take apart and clean though.
Unscrew the valve from the airbrush body. You may have to hold the air valve with pliers to loosen it.
There is a hole in the bottom of the air valve that an Allen wrench fits into. Use an Allen wrench of the proper size, and while holding the air valve body with pliers (there is a knurled ring around the body that works well) unscrew the fitting in the bottom of the valve
**CAUTION!!!** The valve has a very small spring and valve stem in side. Be careful and do not loose them!
The valve stem has a captive o-ring on it. Carefully clean it but avoid getting solvents on the o-ring. The body, spring, and bottom cap can be cleaned with laquer thinner or other strong solvent.
Put a little Vaseline or airbrush lube on the o-ring and valve stem, reassemble, and it should be good as new.
Edit ... It must also be contagious. I got home and my Omni was doing the exact same thing