# 2003 Chevrolet TrailBlazer EXT knocking sound

**URL:** <https://community.cartalk.com/t/2003-chevrolet-trailblazer-ext-knocking-sound/119685>\
**Category:** Maintenance/Repairs\
**Tags:** chevrolet, trailblazerext\
**Created:** [June 23, 2018, 1:54pm UTC](https://community.cartalk.com/t/2003-chevrolet-trailblazer-ext-knocking-sound/119685 "2018-06-23T13:54:18Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![CarlGolden](https://avatars.discourse-cdn.com/v4/letter/c/d6d6ee/32.png) [@CarlGolden](https://community.cartalk.com/u/CarlGolden)\
**Post date:** [June 23, 2018, 1:54pm UTC](https://community.cartalk.com/t/2003-chevrolet-trailblazer-ext-knocking-sound/119685/1 "2018-06-23T13:54:18Z")

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There is a knocking in the very back of my Trailblazer. It only happens when I first start the car. Goes away within a few sec/min. What is it and how do I fix it

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**Author:** ![TXdealer](https://avatars.discourse-cdn.com/v4/letter/t/50afbb/32.png) [@TXdealer](https://community.cartalk.com/u/TXdealer)\
**Post date:** [June 23, 2018, 3:20pm UTC](https://community.cartalk.com/t/2003-chevrolet-trailblazer-ext-knocking-sound/119685/2 "2018-06-23T15:20:05Z")

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Most likely a rear a/c mode or blend door actuator. Sometimes on this generation of GM trucks disconnecting the battery for a period of time will fix the problem, as the computer will recalibrate the doors operation.

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**Author:** ![afadali](https://avatars.discourse-cdn.com/v4/letter/a/b5e925/32.png) [@afadali](https://community.cartalk.com/u/afadali)\
**Post date:** [February 19, 2019, 8:10pm UTC](https://community.cartalk.com/t/2003-chevrolet-trailblazer-ext-knocking-sound/119685/3 "2019-02-19T20:10:54Z")

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Hello there,  
I’ve Trailbalzer 2007, I’ve the same problem above, tried to follow the reset prociedure but it didn’t work, now I need to replace the actuator, it’s the one of the middle of the three on the driver’s side, the problem is I couldn’t reach it from below the steering wheel, do you have a video showing how to reach it?

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**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\
**Post date:** [February 19, 2019, 8:16pm UTC](https://community.cartalk.com/t/2003-chevrolet-trailblazer-ext-knocking-sound/119685/4 "2019-02-19T20:16:51Z")

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Look on YouTube