Thump from dash

I hear a repeating "thump" from inside the lower dash about once a second when my car ('99 E320) is running. I remember seeing a post about this in
the past but can't recall the specific resolution. Something about a heater control door actuator, maybe?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.