Clutch or transmission or other

I have a 2003 Nissan Sentra SER it has a 6 speed manual transmission recently it started popping out of gear while driving and making a sound kinda like a fan hitting something the fan is fine it just sounds like that also when clutch is pressed sound stops sometimes but not always car has 140000 miles never replaced anything on it had opinions from several backyard mechanics ideas like throwout bearing, synchronizers, need new transmission I have no idea and not a lot of money so im trying to pinpoint the problem before taking to shop any help would be greatly appreciated.