Is it directly related to upshifting??
I have a noise that happens when I upshift with a decent amount of throttle and is directly proportionate to how much slippage there is before it(the clutch) fully grabs. If I gently apply gas and release the clutch it doesn't happen. If I accelerate a little harder and there's more slipping before the clutch graps it will make the noise.
Sound like your problem?
Not that I can offer any answers or solutions.. but just be there with you in the same boat..