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My father-in-law gave my boy a 1999 350 Big Bear. It's been sitting in a barn for a bunch of years. It runs great! My boy has been riding a couple weeks now and it has developed a squeal coming from the front end when you get off the gas at speed. Not as much noise during acceleration. At first I thought brakes. I took it for a ride and I don't think it is the brakes. I feel it is the front prop shaft U-joints, front differential or one/both axles. I pulled the fill plug on the differentials and topped them off. I don't think it made much of a difference in the amount of squeal up front.

#1 How do I check or know if the u-joints are bad? That looks like a "Big Bear" of a job if they don't need to be replaced.

#2 What else could it be? I don't think it is the front bearings. I jacked the front wheels off ground and no play in the right wheel. Very little play in the left wheel, looks like the lower control bushing is where that play is coming from. The rubber CV boots look original but all still intact. Wheels spin easily without any noise. On jack stands I cannot replicate the squealing noise.

Thanks for any help!

PS: Here is a short video of the noise:

 

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sounds like brake noise to me thefronts are drum on that machine right? Or some dry and rusty wheel bearings.
 

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To remove the front driveshaft, you need to unbolt the front diff and pull it forward in the frame. The driveshaft splines will then be able to disengage and slide apart.
Closely inspect the u-joints for rough operation and play in the bearings.
 

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Welcome from Colorado.
Bearing can be bad and not felt by hand with the wheel on the bike. I know :)
One time I I removed the wheel and still wasn't sure there was a wiggle, then I removed the axle nut and the hub and the balls fell on the garage floor. he bearing was toast. And if you found any wiggle, that is too much, even if from a ball joint. Your son could be seriously hurt if that things comes apart at speed.
As for the axle joints, sometimes you can barely wiggle and feel a severely worn part, sometimes you have to remove the boot. You can hold the shaft with vise grips to wiggle the tire for the outer joint, and for the inner hold the front brake and wiggle the rear tires to send a twist through the to front. You might find the problem somewhere else in the drive line. I would do these things before pulling the axles.
That bike is old enough to need several systems fully serviced and more than one part replaced.
 
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