i have a 2000 ford f-150 v6 5-speed man. with 106,000 miles on it. i noticed that when i upshift into 4th gear it grinds about 80% of the time, but it doesn't do it in any other gear. it also does not do it when i am downshifting. does anyone know what can cause this and how can i fix this. thanks for the info!
Have you been under the truck and inspected the transmission?
Have you checked and/or changed the fluid?
There are some rubber plugs on top of the transmission that can leak, causing a loss of fluid.
I seem to remember problems with 4th gear being the first sign of a problem with low fluid.
I suggest draining the fluid and refilling to proper level with ATF.
And, look closely for signs of a leak. I have been told the fluid will leak while driving, and this can cause the fluid to blow back, not drip straight down the transmission housing.