Knocking from under dash when key turned to on position

[MrBossUG]

Actuator for what exactly?

Most commonly a blend door actuator. For the climate control system in the vehicle. There’s usually at least one for the blend doors to change the vents it’s coming from, the climate one to switch from heat and a/c. All HVAC actuators opening/closing something. There’s usually 3-4 all the up to 6 (as far as I know)