Normal and like other have said, it is from the fluids moving around and slight transfer of fluid forces. Sort of like the basic of a Automatic transmission, nothing but fluid and if one pulls a torque converter apart there is no physical drive between the drive (engine) and output (Transmission) or more properly between the impeller and the runner, but that fluid will eventual create enough transfer to overcome the force holding it from rotation and start to spin. Granted there is nuetral, but you could put a direct hydraulic coupling (Torque Converter) between engine and shaft and it will not turn while be held until the stall speed is reached and then locked up per say, while still allow slippage.
One of the reason that there is different stall speeds on TC.
