Whooshing clutch noise at temp

[Vanilla]

P911s 1977, 2.7lt- 915 5 speed new clutch 12 months ago. I'm getting a whooshing noise from what appears to be the gear box once the car gets up to normal running temperature. Starts when releasing clutch into new gear, noise last between 2 and 5 seconds and gradually fades out. If I drive the car for along time I start to get the whooshing noise at around 2000-3000 rpm without gear change. There is no sign of clutch slipping etc as the car drives very well. I did replace the engine seal behind the flywheel when changing clutch however, I suspect there is still a small oil leak. All other clutch party's are new except for the clutch fork.

Any assistance would be greatly appreciated.
Michael