1. Should I be able to smell gas after the service panel for the burners is removed?
When the furnace is running, the service panel is not supposed to be removed for safety reasons. For instance, the blower may suck the air and make the burner go off and cause gas to fill up the whole house.
2. There is a musty smell when AC is on, should I call the repairman?
You can use a bleach water mix with water 1 : 1 ratio to spray the evaporator coil. If the musty smell does not go away, then call a repair technician.
3. Sour smell coming from the air ducts when the heat was on, and again as the weather warmed up, when turn it to cool for the first time
The duct needs to be cleaned. Replacing the insulation inside the duct can also stop the smell
4. The furnace exhaust and condensation water smells like raw gas, but there is no gas odor or exhaust gas around the furnace - unless you stick your nose right in the condensation pump - or anywhere inside the house.
It is normal for the furnace exhaust to smell of raw gas.