'83 KZ550 LTD grinding noise 6th gear

Greetings !
It's a 1983 KZ550 C4 LTD with 52k miles on odo.

I have this problem, when I'm accelerating in 6th gear, this grinding noise happens,
loudest in mid-range RPM's, almost no sound in high RPM's.
All other gears are completely fine, this happens only in 6th gear...
New: oil, filters, spark plugs, battery.

Any recommendations, advices, ideas, solutions ?

Thanks.