I had a similar problem to this on my 4x4 saph. When in 4th gear and on overrun I could hear a grumbling noise coming from the transmission. As soon as I accelerated or dipped the clutch the noise would stop.

Tried to pin point the nosie with the help of a couple of mates, they both swore the noise was coming from the rear diff. Had diff rebuilt by DOME and new rear CV joints on the driveshafts, also changed centre bearing on rear prop but grumbling was still there. Gave up in the end and just lived with the grumbling noise for 6 months.

Last month I changed the front wheel bearings and the grumbling has now completly stopped

Going to change rear bearings next month just to be on the safe side.