I have a 95 with the 4.9L straight 6. About 30 seconds after starting a serious knocking sound starts with vibration. It lasts until the truck warms up. This is causing the clutch to act up I think. It has been leaking a small amount of fluid, that seems to increase when it is acting up. While driving when it's still warming up I find myself having to take the truck out of gear to avoid the vibraion and knock from getting worse. I was told about a year ago my syncronizers for the transmission were worn. Now it won't let me shift while the truck is running, although when it is shut of I can shift with no problems....

Any ideas??

 

You'll need to open up the tranny and inspect it. Stick right? When mine was knocking it was a chipped gear. You may have shattered a syncronizer and sent pieces all over inside. You can pull the top off the tranny without removing it. There's a large floor plate where the shifter comes through, about 12"x14". You'll have to pull the carpet back to get at it and maybe pull the seat, but that's easier than pulling the tranny. Of course if there is internal damage then it will have to come out anyways.