While using 2 microwaves and a toaster oven (kitchen) I walked by the circuit breaker box outside and noticed a periodic "hum" or a "buzzing" sound. Every 30 sec or so but
It never tripped a breaker. It was not very loud. Rarely the kitchen circuit will trip its breaker (1-2 times a year) but I've never been outside near the breaker box when it has been tripped. The house was built in 1975 and has had no electrical issues. What made this noise? And what to do about it?

Replace the circuit breaker and check all connections in the panel.