I take it you have an LS400, This is a common problem on this model, if the fluid is full and you have no leaks, then it is likely you will need a new pump etc. This can cost a lot of money, there is no point in buying second hand as you will have the same problems. Even if the fluid is slightly low you will still get a whining, they have to be spot on the level.
Depending on how much you love the car and how long you want to keep it and how much money you want to spend on it, you could replace it, or you could just live with it.
You could try to flush the fluid > Take a look here
It is possible to re-build the power steering pump on the LS400, Detailed instructions and a photo-tutorial can be found here. hope this is of use to you.
Bison :zee: