Question about 1994 Chevrolet C1500

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Thumping sound under dash when start or shut down.

When I turn on key the sound starts, and thump- thump- thump- thump-
goes for about 15 seconds then quits. Then when I turn key off, the same set of noises, goes for about 15 seconds.
It is located under the dash, about middle of dash.

I have looked under there and have tried to feel where the thumping is at but I cant find it.

It sounds like a flapper door shutting.

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  • llswade1 Nov 29, 2008

    The engine does not need to be running. Just turn key on....noise
    turn key off....noise again.
    Engine is a 5.7


  • llswade1 Dec 01, 2008

    Thank you Shouldibuyit.
    I removed the fuse and the noise did not start when key was turned on or off.
    Put fuse back , and noise came back.
    Sounds like a door slamming shut over and over.
    Thanks, Larry




  • Don Minor
    Don Minor May 11, 2010

    When you turn the key on, do you mean engine is running. and what engine size do you have?

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I take a shoot here you can try this. try pulling the fuse or fuses that go to your heater blower and heater controls. If it stops check your heater blower it may be going bad, if you find nothing there, leave the blower unplugged and try turning your key on and see if you still hear it. If so I would think that the heater control is mess up and open and close the air blend door or you may have a wire shorted out under the dash some where.

Posted on Nov 29, 2008

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