Noise from behind dashboard

[Mr Hugo]

I have a noise from behind the dashboard that sounds like a
vent opening and then shutting constantly, it only happens when the heater controls are used, (Auto air con, or blower)
If I switch the controls off it still continues to make the noise.
It must be some sort of vent I am sure.
The car is a 1996 328i saloon.
A big thanks in advance

[Chilly]

Don't know about E36. My E30 had a rod linkage come unhinged from a damper flap. It was the flap that selects inside or outside air. But it didn't cycle all the time. While reconnecting it I wondered how the little motor knew when to stop moving. Was there some sort of limit switch integral with the motor? Did the flap trip an external switch that shut off power to the motor when the flapper reached full throw? Still don't know, of course. But I wonder if whatever tells the motor to stop turning isn't getting the message to your motor. I suspect these flapper motors are very similar to wiper motors. There's some magic going on in those wiper motors that cause it to always turn off at the bottom of the windshield. Without that little bit of magic they'd simply shut off wherever they are when you turn the switch. For wiper motors that bit of magic is integral to the motor. Probably the same for your flapper motor. If so you may need to replace the motor.

I hope someone can confirm or correct my assessment. You may end up deep in the bowels of your dash. Wouldn't want to do that on bad information.

Chilly

[Mr Hugo]

Thanks Chilly for your advice, I shall look into it.
I did take out the left hand side glove box to look for the cabin filter (yes it was on the other side I know now) UK spec
I might have unhinged something there.