Throttle Creak

There's a creaking sound when easing on or off the throttle of my '88 300SE. I don't think it's a problem with the linkages, because there is no creak if I ease on or off the throttle with the car off or in N.
Any ideas?

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.