We had a new a/c heat pump put in last summer. On and off, we notice a smell from the vent, only when it start either cool or hot air. This is an electric unit, so no gas.
Any idea on what the smell might be? What to look and check.

Thanks.

Usually most smells come from bacteria on the indoor heating/cooling coil. The coils can be cleaned by a person in the HVAC trade. I would contact the company that installed your unit and complain.