Auxiliary heater?

[Graham couch]

When I put my heater on in my Slk 200 I can hear a wirring noise, it's not the fan but a noise that stays on after the fan is turned off, and it does get warm quick, is this an auxiliary heater? If so how does it work?

 

[Craiglxviii]

It won't be an aux heater, they're a specific unit. MB petrols DO warm up within a few minutes so I suspect it's a warm air bypass system.

 

[Graham couch]

Oh ok thanks