vent system problem

1993 GMC SIERRA
200,000 MILES • V8 • 4WD • MANUAL
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JOHNR2610
  • MEMBER
  • 1 POST
I have a 1993 GMC 1500 and the dampers on the heating/ AC system make a thumbing sound when starting or stopping engine or switching damper positions. What causes this and more importantly how can I stop it the annoying noise? thanks, John
Jan 3, 2011 at 9:35 AM
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JACOBANDNICKOLAS
  • ✓Experienced Mechanic
  • 110,246 POSTS
Chances are one of the mode doors is sticking. All you can do is craw under the dash, locate where the sound is coming from, and from there determine what is causing it to stick. If you hear it when going from ac to heat, chances are it is the blend door next to the heater core.
Jan 9, 2011 at 3:41 PM
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