But it's been running since Friday jumped. If the compressor was going out....but I will be back on tues.Try this. I had this happen to me once. A compressor on a unit was on its way out and over amping, but not enough to cause the breaker/fuse to trip. When it started it over amped it caused a voltage drop to the unit. 208 3 phase dropped into the high 100's(don't remember exactly how low it went) this caused the transformer voltage to drop also because it was being supplied less than 208 which caused the contactor to drop out because it was seeing in the high teens for a voltage.
It happened so quick, my meter wasn't picking it up. As soon as the compressor started the voltage dropped and caused the contactor to drop out and chatter.
To test this, put your meter on the transformer and push the contactor in and see if the 24v remains constant. Also, check the compressor amps and meg the windings if you have a megger.
Unit had a failing compressor. Changed it out and of course the chattering went away. We also got an electrician fix a problem with the line voltage wiring to the unit adding to voltage sag.