Don't use 5. gear.
Crogwheels, bearings or shift-fork service ahead.
What's your milage?
I concur. Something is wrong. It would be nice to enjoy the Summer. 5th gear is tacked on the back of the case. Hopefully by not using 5th You can get thru the Summer. A rebuilt T-9 might be an easy fix. Unless You can do the work, I suggest You take it to a transmission shop so You do not have to worry about this noise.