I noticed my outside unit buzzing the other day, and the fan was not running. The unit is about 7 years old. I changed the capacitor yesterday, and the unit worked for about 5-10 minutes, but then shut down again with the fan stopping and the buzzing again. I'm 99% sure the buzzing is coming from the fan motor, so that was my next guess of what to change.

Does that sound like the best next step? Should the new capacitor still be okay? At one point I think the pressure release valve opened because I heard a loud hissing. Does that mean some refrigerant escaped?