Technical 3 speed c10 - Low speed noise

Out of the blue I noticed a clicking noise coming from underside of my truck. It's not a clanging noise but more of a loud click.... think card on bicycle spokes but louder and heavier sounding. Even at low rpm the clicking is fast and it speeds up until I get to probably mid-second gear then goes away until I downshift and slow down (3 speed manual). It's much louder outside the truck than inside. I don't feel any different vibrations and it drives the same as always. I didn't see any problems just by eyeballing the underside of truck. Any ideas where to start? Thanks in advance.

 

Chock the wheels, put it in neutral, crawl under there and feel how tight the U joints are, and the center support if it has one.

does the noise change with letting off the throttle? is it as loud when you're driving at constant speed, or accelerating?

 

I was thinking it might be u joint but when I went under it there wasn't any play I could tell. The noise never gets softer or louder. It just slows down a little as the truck slows (or speeds up but then stops before I shift into 3rd. Slow down, downshift and it starts again. There is little fluid leaking from where the transmission meets the joint but not much.

 

Bad u-joints aren't necessarily loose. They are often just 'tight'. Run them that way long enough and they do get loose.

While the u-joints were my first thought, I also had a wondered about a chipped gear tooth in the trans......

Ray

 

It was front ujoint. Both were worn so replaced front and back. Thanks everyone for the feedback.

 

The hanger bearings on those were spectacular when they self-destructed. After one going on me, that was the 1st thing I changed when I got another truck.

I had one beat the **** out of the underside of the bed and rip out all the wiring to the back of the truck and wrap it around the driveshaft. A definite treat at night and being 1/2 in the bag.

 

Suddenly appearing strange low speed ****** noises in a RWD GM? I always look for an oily, soft rear trans mount first. They get soft enough that the outer, U shaped steel part gets close to, or touches the crossmember. In automatics, you hear the rear planetary in low range. Just put a mechanic's bar between the crossmember and trans and try to lift. If the mount is greasy or oil covered, just change it before it takes out the front mounts [and fix the oil leak.]