transmission humming

[kartone]

Due to a diff problem (broken tooth) had the diff and while at it the transmission rebuilt last year.
Since then there has been a humming noise from the transmission in 4th gear only, more pronuonced while cruising (no load).
Any inkling as to what the source can be? Many thanks.

[kartone]

Thanks Sportiva. When I'll get a chance I'll have it diagnosed by the the workshop that did the repair / reconditioning of the transaxle.