Rattling noise in 5th gear

[aceflyer]

I've recently been getting a rattling/vibrating noise from the engine bay when driving in 5th gear. This noise doesnt occur in gears 1-4 and also disapears immediately if I drop down from 5th. There are no noticeable performance issues and car accelerates fine in all gears. I've searched everywhere but am unable to find any possible reasons for this noise. Any suggestions?
Thanks.

[taurus_tem]

Possible worn gearbox? would get that checked soon as, my old mitsubishi was doing the same thing, gearbox died.