Work truck vibrates intensely at a certain rpm.?

Why would my lead man's work truck start to vribrate as if he was on the side of the highway hitting the bumpy portion meant to wake you up if you fall asleep. It happens when he drives straight so I don't think it's axels. It only happens sometimes and if he takes the foot off the gas and floors it then the vibration stops. Its a very strong vibration and I would swear he was on the shoulder of the road when it happens but he's always in between the lines where he Is supposed to be when it happens. Is his transmission shot? 

Update:

I have had experiences with mud being stuck in the rim and it isn't that. That is something that will cause the truck to always vibrate when up to speed same with wheels off balance. This is a deep grumbly vibration that happens intermittently and not when the truck is up to speed

4 Answers

Relevance
  • 1 month ago

    Engine mounts shot.

  • Anonymous
    1 month ago

    Many causes, nothing simple.

    You need to run the vehicle on dynamometer.

    A missing or loose Body or Engine Mount.

    Abused engine, bearings going.

    An unstable harmonic balancer.

    Broken or chipped Engine Cooling Fan.

    Out of round Tyre, or wheel Rim.

    Wheel bearing going.

    Brake drum/Rotor that is out of round.

    Loose or defective Belt,

    Timing chain (rarely).

    Crossed spark plug wires, shorting.

    Damaged Automatic Transmission Plates (rare).

    Parking Brake not disengaged properly (very rare).

    Overheated, Warped Brake Rotor/Drum (rare)

  • 1 month ago

    Wheel(s) out of balance..........

  • CB
    Lv 7
    1 month ago

    Out of balance wheel/tire (mud stuck in rim or thrown a weight), tire belt twisting or tread starting to separate, loose suspension parts etc. Take it to a truck front end specialist and have them give it a once over for problems unless you can jack the truck up and do the inspection yourself some confidence. 

    Update - U-Joints - a motor mount you could test by putting it in park have someone watch the engine and give it a quick rev - it the engine lifts up and slams down that is mounts.  Jack it up and start spinning s#it - see what is noisy.

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