Vehicle runs fine but when transmission is deciding to change from 3rd to 4th gear at 35-42 mph, I get a vibration. I manually move shifter to 3rd gear and the gear problem goes away, but if I leave in 3rd gear in drive trouble never happens,

I originally thought a heat shield may be vibrating but could not find, I did see on internet that it may be a waze plate in transmission, annoying but harmless to vehicle. I don't have a warm fuzzy about it.