Question about 2006 Chevrolet Trailblazer

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2006 Chevy Trailblazer loud thunk noise at start up

When I start the car, before going into gear there is a loud ''thunk'' sound coming from the front right hood of the car. My chevy gargage has heard the sound, but can't tell me what the problem is...

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  • Anonymous Mar 07, 2009

    it happens before shifting into gear. When sitting in the driver's seat, it sounds as if it is coming from under the hood, if front of the passenger seat

  • kevans706 Mar 07, 2009

    The last post was by kevans706, not a new user. couldn't figure out how to answer the question asked of me.

  • kevans706 Mar 10, 2009

    Ok, now I am being told that it is coming, the thunk, from under the dash. Isn't the Air unit housed under the dash? It is not a 4 wheel drive. Help! It is happening more often and more often.

  • kevans706 Mar 10, 2009

    Ok, so I have turned the air off, and started the car twice....no thunk. I have another appointment for the SUV next Tuesday....if this contiunes to not happen when the air is off at start up, what do I tell my auto shop when I take it in for repair? What should be repaired?

  • kevans706 Mar 11, 2009

    Thanks, I am going to print this and take it to the garage next week. I will be back on to let you know. Kevans

  • Fahr Quad May 11, 2010

    Does the thunk happen when starting, before shifting into gear, or after shifting into gear but before it engages? Is that noise on the front right from the drivers seat or standing in front of the car?

  • Anonymous Mar 28, 2014

    there is a loud screeching sound coming from one of my pulleys, how can i tell which one it is

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My first guess would be the front u-joint on the drive shaft. Try twisting the u-joint to see if there is any play, which there shouldn't be. If the u-joint is OK, have the transmission checked out at a reputable shop. There could be a problem with the torque converter, the valve body, or the bands. You didn't say whether this was a 4 wheel drive or not, but I doubt that the problem is with the transfer case. Let me know what you find.

Posted on Mar 10, 2009

  • Fahr Quad Mar 10, 2009

    If it is under the dash, it could very well be a vacuum operated damper in the "heater box". Try switching the heat and air off, restart the vehicle, and see if it happens again.

  • Fahr Quad Mar 11, 2009

    There is a door on the heater box that is opened and closed using a spring loaded hinge and a vacuum actuator. You may have a leak somewhere on the vacuum lines allowing the door close when the engine is off, and when you start the engine and build up manifold vacuum again the door opens suddenly. The vacuum leak can be checked out by using a vacuum gauge and checking for leak down, or you can run an unlit propane torch around the vacuum lines to find the location of the leak. The engine should speed up when you are at the leak.

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