trasmission

i have a 5 spd and i hear grinding and sometimes the gear pops put, is only happen four times, sometimes i shift gear and it seems like the clutch isnt i even push even though it is, its really worried me cause i know how transmission cost alot and well any help would be greatly appreciated, also my truck has just reached 140,000 miles

Check the simple items first. Check that the fluid level in the clutch master cylinder is OK, if it needs topping up use DOT 3 brake fluid. If it needs topping you should look for signs of leaking at the clutch MC and the clutch slave cylinder on the bell housing. If the hydraulics have ever been too low you may need to bleed at the clutch slave cylinder. Take the truck for a drive and find a hill with the transmission in fifth gear can you detect any clutch slippage? If it slips you need a new clutch. Check the oil level in the transmission. If it has never been changed now is a good time to drain and refill. Usually clutch hydraulics problems will be first noticed when trying to get the transmission in reverse or first gear. If all is OK check the condition of the transmission mounts.
If all of the above checks are good you probably have an internal transmission problem. It could be shifter linkage or shifter forks. The noise shifting to fifth gear may be caused by a worn synchronizer.

the fluid level is at the low level line so tomorrow im going to the auto shop to get dot 3. thank you and hopefully this will cure it