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Is the fan in top running?
Is the humming coming from a relay in the outdoor unit or is it the compressor or fan humming?
shut the disconnect off next to unit...tell me if it still hums.
this tells us the contactor is pulled in from thermostat signal.
Now can you follow the 220 wires into unit to the humming part ans see if 220 v is there. Do not measure to ground but measure wire to wire.
If this is the first time starting this spring it is common to trip the breaker. The compressor has been sitting and just needed to get jogged.
I wouldnt worry unless it trips again in the near future.
Thanks for the help on your end!