What causes a sceaming sound in my power steering?

like when I turn the steering wheel in a stationary position it screams like a fanbelt does.
its old school Nissan homy van so the answer should be simple?

Loose power steering belt/fan belt. Tighten it or replace if worn. End of noise.

That'd be the drive belt. Needs adjusting up or maybe replacing.

Have you seen your cat lately?

older powersteering when you fully lock will stress the belt and pump and wear the belt out as there is no pressure releve system i would say the old homy is such a system as are older toyota designs.
once the belt has stress worn it will do it more and more easy , Do not tighten a worn belt you only do damage not fix the problem a slipping belt has to be binned and a new belt fitted no matter how good you think it still looks.
so check belt tension if its tight replace it as its worn. also be awear it might be your alternator belt doing the screeming not he power steer same thing alternator belt usually scram when you start the car in morning cold due to the load the alternator creates to replace the power in the battery that was used on cold cranking . as a indication for what belt screams
also you should check for power steer fluid level and colour of fluid as check