rhythmic scratching sound in Trailblazer

monzamessMay 4, 2005

This is a weird one regarding my wife's 2002 Trailblazer. She has heard it on and off for weeks and I finally happened to catch the noise.

It is a rhythmic scratching sound about 2 times per second coming from under the driver's side dash. Kind of a "ch ch ch ch" sound. The sound repetition stays the same speed regardless of engine speed. The sound comes and goes; usually, if the truck is restarted, the sound goes away.

Weirdest thing: If I turn the lights on, the noise stops for a second, then restarts. I suspect the drain on the electrical system stops whatever is causing the noise temporarily.

Any ideas what's going on?

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If it was a relay cycling, some system would be dancing to the same rhythm. I suspect a gremlin. You could put a meter on your system, and see if the voltage varies in sync with the scratching sound.

    Bookmark   May 4, 2005 at 9:52PM
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On those vehicles, the HVAC system dampers are run by electric motors rather than vacuum. One could be sticking. You might try moving the "warm-cold" knob to a different place as well as changing the setting of the "mode" knob. For example, if it's set to send air out of the dash vents, move it to floor, or vice versa, to see if either of those moves stops the sound.

    Bookmark   May 5, 2005 at 5:36AM
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I think cowboyind has it, the noise you are hearing is the little plastic gears slipping past each other on the blend door inside the HVAC case and each time it does it, more teeth are disentegrating. You will probably start to have problems getting the AC/Heat controls to work properly, I would have my favorite dealer pull the TSB's on this vehicle There is probably one that fits your problem to a Tee.

    Bookmark   May 5, 2005 at 12:35PM
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Thanks for the tips. I've advised my wife to try changing the HVAC settings next time she hears the noise. Unfortunately this one is a tough one to troubleshoot since she drives the vehicle about 95% of the time, and the nosie occurs infrequently...

    Bookmark   May 6, 2005 at 8:07PM
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I surely hope that all this "scratching" noise is not mice who have sought the warmth and security of a modern motor vehicle..It is not easy to make a vehicle vermin-proof..

Would not "mis-engaging" gears make a soft click-click noise ??

    Bookmark   May 7, 2005 at 2:50PM
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If it's mice, they have much better rhythm than I do! :)

    Bookmark   May 8, 2005 at 8:37AM
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