Off-throttle noise!

My 2000 360 F1 is making an abnormal, loud noise when going off the throttle/coasting. It's a mechanical, slightly high-pitched wail which decreases as the speed (and RPM) drops, almost reminiscent of a supercharger although with a more "grinding" sound. If I put the car in N when coasting, the noise disappears immediately, but comes back as soon as a gear is selected. This is only happening when coasting, the sound goes away immediately when accelerating. The sound does not appear to affect the performance of the car (or tranny) in any way, but I am reluctant to drive it until I'm sure what it is. The transmission was rebuilt approx 5000 miles ago. My initial thoughts was about the differential or something with the PIS-setting for the clutch, but I don't know enough about the clutch/PIS adjustment to know if it can cause that kind of sound. The car also had an annual service recently. any thoughts on the matter are appreciated!

 

Thanks. Ring and pinion sounds plausible. There are no visible leakages as far as I can tell, but I'll have to do a more thorough inspection when I've got more time. If this is the cause, what course of action would you advice? Just replacing the ring/pinion and making sure the heat exchanger is in prime condition, or is does this problem usually involve more components? As mentioned, the gearbox was overhauled/rebuilt a few years and miles ago. I happen to have an "engine and drivetrain insurance", so I guess it is time to see if its worth its salt

 

I spoke with my local specialis, he's had some dealings with this problem earlier. The insurance company has also contacted them. It looks like the local specialist will drain the gearbox and take it from there.

 

It was the ring and pinion. My specialist will source a rebuilt box while my insurance company will cover the bill