Servo on the fritz?
 

My w116 77 450 sel may or may not have an issue w/ the AC servo unit (I've also seen it called a climate control valve). I bought the car w/o a hood cover which would suppress engine noise, but the unit seems pretty loud from the passenger compartment.
When I first heard it I thought it was the fuel pump, but I hear it coming from the firewall. Then I put a stethoscope on the servo and that was where the sound was coming. The sound is intermittant, coming on for 30-90 seconds and then turning off. And it also comes on and off whether I've got the climate control system on or not. Is that normal?
My Climate Control unit seems to work fine, air blowing cold and heater blowing hot. But that servo sound is kind of annoying. Do I have a problem w/ my servo, or do I just need to get myself a hood cover?