Knocking Under Dash


Asked by Apr 18, 2016 at 09:25 AM about the 2003 Chevrolet TrailBlazer EXT LT 4WD SUV

Question type: Maintenance & Repair

My truck recently wouldn't start. My husband found that the battery
connection was loose. As soon as he fixed it a loud knocking started from
under the dash. It happens everytime I start the truck and lasts for about a
minute. What could it be? Thanks!

18 Answers


That sounds like your fan motor! Does the noise under the dash go away after about a minute?

3 of 3 people found this helpful.

Check the oil in the engine. May be low on oil, or you may just have a knock until oil pressure builds up.


My dads truck does the same thing. It knocks for about a minute an goes away! It's the Fan Motor! Go to the passenger side of the car open the door an listen up underneath the dash right under the glove box an let me know if that's where the Noise is coming from?


I don't believe it is the oil. It was just changed a few weeks ago and the knocking is not under the hood. Yes, the knocking stops after about a minute. It is in the middle of the dash, not on a specific side.

1 of 1 people found this helpful.

The thumping knocking noise is your Fan Motor! When you turn the Engine off does the Thumping Knocking Noise happen?

1 of 1 people found this helpful.

New Fan Motor will fix that Problem!


It doesn't knock when I turn off the engine. How do I get to the fan motor? Will I need to buy other parts? Thanks!!


Just trying to look up the part on ebay motors and there are several options. There is a blower motor heater fan or an a/c heater fan blower motor resistor. Which do you think I'd need?


Call your local parts stores an get a quote on your Fan Motor!

1 of 1 people found this helpful.

Blower motor an fan motor are the same thing


As Fiemfam7 is saying, there looks like there is two blower motors. One blows for the front and one blows for the rear.


Ok, your vehicle has 1 blower motor! That resistor is just a switch! You need a Blower motor for your heat, an AC.


Flemfam7, go to YouTube an type in 2003 Chevy trailblazer blower motor! It will show you how to fix your problem. I just searched it for you.


You guys know that there just may be some debris, or an alignment issue that is making the squire cage to hit or rub against. I would just take out the blower motors and look to see what the squire cage is hitting. If it was the motor itself it would have started to squeal first.


Squirrel Cage....


It is a HVAC Actuator. Easy fix.

1 of 1 people found this helpful.

My 2002 Trailblazer is doing the same thing. I have not looked into it closely yet and will check the Blower Motor, but my first thought was a an actuator that is stuck and trying to change the air direction. Here is a video I found on YouTube for the actuator.

1 of 1 people found this helpful.

Blend door actuator, I replaced mine and the problem went away.

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