Toyota Land Cruiser 105 Transfer case howling in low range

[LRT]

A relative of mine has recently bought a 1999 Toyota Land Cruiser 105 with the 4.5L 1FZ-FE petrol engine and 4 speed auto.

When in low range the transfer case howls at any speed.

However, it drives in high range without any abnormal noises.

Does anyone know if this is due to any of the following:

The transfer case design uses straight cut gears in low range?

Lack of oil or incorrect oil viscosity?

Bearing failure?

[LRT]

I noticed that our Discovery II howls in low around 40kph, so maybe it's normal? The odd thing is the Land Cruiser howls virtually from standstill and the auto is jerky when up shifting. It is a world apart from high range.