97 E320 Whistling noise
 

Recently my 97 e320 developed whistling noise comming from the glove compartment or below.
I removed the glove compartment and removed the filters, it is not the motor noise.

When the car is not running and but the key is in on position with the heater and fan on I hear the vacuum pump starting every 30 sec and going for 5 sec.
Also when I drive the car it has the same noise at different speeds and rpm.
At the same time I hear very loud whistling noise from under the glove compartment, it sounds like some massive vac leak.

How can I check it and what do you think could be the problem?

Greatly appreciated

What is the best way for me to pinpoint where is the air leak (which I suspect) or what is this whistling noise from under the glove compartment?

Thank you for your help