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When I shift the truck into 4Lo there is a loud clanking noise from under the car. It is really loud and sounds like someone hitting a pot with a hammer. Any ideas?
 

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When I shift the truck into 4Lo there is a loud clanking noise from under the car. It is really loud and sounds like someone hitting a pot with a hammer. Any ideas?
it's normal to hear a moderatly loud "clank" when going into 4lo. All my GM 4X4 trucks did this.

I did have a truck that had a bad u-joint. The clank sounded much louder and hollow.

Are the rpms a little high at idle?

cheers!
Eric
 

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Sounds like U-joints, go underneath the truck with a grease gun and grease the zert fittings for the u-joints.
Most newer vehicles only have a few grease fittings it's all sealed systems.
Oil change centers have for the most part stopped lubeing the undercarriage. GM uses a lot of Spicer u-joints which require greasing. Look at them very carefully, at times people will say the joints dont have fittings and cant be lubed when in fact they probably have never been lubed and require the fittings to be screwed in before they can be lubed. (most serviceable systems dont come from the factory with a zert fitting installed).
 
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