Outside unit making low constant hum noise, can be heard 4-5 ft away. fan on outside unit blows when thermostat is lowered and ac comes on. Cool air blowing from vents in house when air come on. Wondered what the noise is?

Not at all uncommon. What you are hearing is a hum from the contactor. Sometimes a simple, firm rap on the outdoor unit where the wires connect will cure it. If not & if it is really annoying, replacing the contactor should cure it. The hum will hurt nothing.

Thank you