You are likely hearing the fan running on the a/c but the compressor isn't starting. Or it shuts down quickly due to a problem. Without some more detailed symptoms, it is nigh impossible to diagnose. Freon leaks in roof a/c units are not common and not the first thing to look for. Still a possibility, though.
When the fan cycles on, you should hear a deep Hummmm within a few seconds. Loud and then smoothing out to a quieter hum with a few seconds. That's the compressor starting and running. If you hear a loud grunt which just stops, that's the compressor failing to start. If you hear neither, it may be that the control board isn't even trying to start the compressor.