Seems like when I hit 4th gear running 70 mph a vibration/ humming accurs. When I let out of the gas it goes away. Anyone know what it is

 

brass synchros wearing or shift forks/gear set wear in the transmission..would not be overly worried, unless it gets worse....could be the onset of cold weather....in the old days we would swap out trans fluid this time of year for lighter weight in the winter...

Pistol grip shifter on a mopar 4 spd is hard on forks. Had them break twice in the same winter till I got the 90weight out of the trans..

YMMV

 

my dad owns a tree company. we have to do the same In the winter because the heavier weight trans fluid puts more strain on the trans. Idk if it is because the colder weather makes the t fluid thicker or what but I would definitely say that's what it sounds like. its worth a try at least.