Ringing/rattling Sound In 6Th Gear

[Alhazred]

I have this very concerning sound in 6th gear,kind of a ringing/rattling sound.At around 4000rpm it's constant and quite noticeable,when I back off to 3500 it becomes intermittent and almost disappears approaching 5000 it is drowned out by the exhaust or disappears.It stops when I disengage the clutch.I can hear it very very slightly in 5th gear but not enough that it would bother me,when I click up to 6th though it really bothers me I have this nagging feeling and it's hard to enjoy the ride when i think something is wrong with it. Anyone have any ideas?