After about 3 weeks of working properly, our new AC unit outside began to make a noticeably loud buzz as though the compressor is trying to start and won't. The noises are random. The unit makes a lot of that same noise when the system is turned on but does run seemingly okay at first. Set to 78, the AC cools for about 5 minutes then stops (fan still blows) and this loud buzz begins. The outside temp is hot these days in the 90's. The thermostat is set to 80 for now and it MAY lower the house 1 degree until the above scenario begins. What might be making this compressor (that does work at times) shut off so soon and not re-start but make gigantic buzzes in what seems like an attempt to start? Thanks for any advice.

It is under warranty if it is new. Do not attempt any repairs yourself in fear of voiding warranty. Call the installer to schedule service.