Weird grinding noise in 4th gear.. HELP

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Hmm, fourth gear in these transmissions is the direct drive (it's that way in most transmissions.) This completely bypasses the countershaft in the trannie. My guess on this one is that one one of the bearings inside is starting to get a little weak. What year is yours? 86-88 B2000 used open bearings and 89-93 used sealed bearings. The open bearings are alright and will last as long as the oil is maintained. When was the last time that you changed the fluid in your trans? what type do you use?

EDIT: just saw that yours was an 87. still try the above fluid change if you haven't already. lemme know if you see any metal shavings.

Thanks for the Input. and yea its an 87. Havent had a chance to do really anything to the truck since i got it. (about 3 weeks) its been nothing but snow ice and rain since. but as soon as it clears up ill change it. and now that you mention it it does sound like a bearing. lol

Well from my experience the factory oil that is used in the transmissions is a little too thick for colder climates. What I have found to be useful is to use ATF in our trannies. This is thinner and you won't be grinding gears as much in the cold. I know several people whom have used ATF over 75W90 in their trans without any issue. I ran for 2 years before the engine died. Trans looked good on the inside when I regeared it too so

Ill have to try it.