feyguy

I am new to this site but hopefully I can find an answer soon. I bought an 85 f150 from a guy on craigslist in october. It is pretty beat up when it comes to the body like rust and stuff but the frame is solid and the engine runs strong. Recently, the real differential has been making metal on metal sort of grinding and clunking noises when driving. Especially on shifting from drive to reverse it clunks and makes a metal ding noise. I can't seem to find a pattern and sometimes it makes the noise when cruising or coasting. I will accelerate and it will make the noise too. I have taken a look at the gear oil level and it was low but I bought some oil and filled it up and no luck. Can it be u joint or other connections or is this something bad with the gears inside. I cant really take off the cover because Im afraid of not being able to put it back on. Also the bolts are on the front side near the driveshaft and will take a lot of work to just take it off. I am suspecting bad bearings or something specific like a pinion bearing or something like that. Anyone else have a problem like this? Hopefully I can get an answer.

wrvond

Yes, it could be u-joints. Lay under the truck and rock the shaft back and forth. Visually watch the u-joints and listen for the source of bad sounds.

yokev

A bad U-joint will allow slack when you take off from a stop, then engage. You can check 'em by getting underneath and simply turning your drive-shaft back at the U-joint and look for slack. Grab the d-shaft on both sides of the U-joint and twist one while you hold the other still. If you see a fair amount of slack, chances are you have yourself a bad U-joint.
HOWEVER, U-joints don't really make 'grinding' noises, and obviously once the drive-shaft is spinning, they don't tend to 'show' any symptoms as the slack is already taken up.
From what you wrote about it, I'm seeing damage INSIDE your diff.
You'll get the same 'symptoms' as you will with a bad U-joint(slack), but also may hear metallic sounds such as GRINDING, and will continue to hear and possibly even feel it while your driving. Since the gears are spinning all the time, it really doesn't matter if you're on the gas or not. You might have a bad U-joint, but that's secondary to what's going on inside your diff.
You're going to have to pop it open to find out the extent of the damage.