It could be the " center viscous coupler" VCD driver mentioned in his post. Here are some of the symptoms:
Another problem associated with the viscous coupling is the difficulty of accurate diagnosis. A faulty viscous coupling might produce a range of symptoms, such as shuddering in turns, or what sounds like bearing noise, and these are often misinterpreted, even by experienced mechanics. A customer might pay for replacement of items such as the tail shaft, power steering pump, steering rack, one or more wheel bearings, the rear differential assembly and even the gearbox assembly, without fixing the initial problem. The viscous coupling is often the last component to fall under suspicion.
Owners and repairers sometimes think they might fix a problem with a binding viscous coupling by changing the gearbox oil, or putting in additives. Unfortunately, while the viscous coupling does run in the gearbox oil, the problem is either with the liquid silicon or the case hardening inside the coupling shell. These are entirely separate from the gearbox oil.
Customers often report mysterious symptoms that prove to be problems with the viscous coupling. Because this component is hidden and works in the background owners are often baffled by the symptoms, which sound like they could be coming from one or more of several drive line components. Sometimes customers waste money on unnecessary repairs, One customer noticed a clunking sound while parking his Subaru Forester at low speed, and consulted several people to find out what the problem was. His local workshop was not able to diagnose the problem, but another Forester owner in an internet chat room suggested it might be the viscous coupling, which proved to be the case.
Another customer reported a more alarming symptom, “chattering” in the front wheels while turning, felt through the steering wheel. The customer suspected the front CV joints and replaced them along with the axles. No change. The problem was worse when cornering under power, but still present when coasting though a turn. The customer suspected a range of other components, before bringing the car to us, where we diagnosed a faulty viscous coupling. Many owners don’t consider this part as it is hidden, does its work quietly in the background, and when failing produces symptoms that seem to implicate other drive line components.
Another customer complained of a “whirring” noise that seemed to run throughout the car. This noise was present both under power, and on a trailing throttle and was louder at higher speeds. Because the location of the noise could not be isolated the customer was baffled. The customer was advised by one workshop that it was a wheel, bearing, which he had them replace. This did not fix the problem. We were able to diagnose a faulty viscous coupling bearing plus input and output shaft bearings.
If you want to read the full article https://www.alldrivesubaroo.com.au/gearbox/subaru-centre-differential/
