Are your CV boots looking okay? You should inspect them for cracks or tearing. Sometimes they may look okay but if you squeeze them some fluid may find its way out of the shaft. And if you see any of those symptoms, you most likely have an axle that needs to be replaced. Sometimes a torn CV boot can be replaced and the axle will be restored, but only if you catch it early on before you hear any sort of noise from it. When a CV joint loses its lubrication, it basically becomes a metal on metal joint. And once you have metal on metal, the joint will quickly wear itself out beyond reasonable repair. Also when this joint heats up after driving, it will cause all kinds of strange sounds coming from the rear end which I think may be your case here...

Not too long ago, my 82' 4 speed had a torn boot in the right axle that I was aware of. I think during the previous ownership it had ripped and leaked most of the grease out. I let it get worse knowing that the axle was most likely a goner anyways. The right axle of course started clunking around after a year or so. I distinctly recall pushing in the clutch and coasting caused the noise to die down. I ended up replacing both rear axles with some aftermarket ones. The noise went away, and the car drove nice and smooth after that. But boy some of those aftermarket axles are absolutely junk. Check those CV boots and let us know what you find.