Resonance at 65mph --- any clues

I've got a 5 speed and have just developed an annoying resonance/hum at 65mph in 5th gear. If I put the slightest of pressure on the gear shifter the hum goes away. Driving at 60 or 70 mph makes the hum go away.

Is this a bearing problem in the tranny or a CV problem?

Thanks

Maybe check the trans fluid level. If you have a lot of miles on the fluid, maybe you ought to just go ahead and change it out.