Welcome!
Sadly it could be plenty of things so there is no simple answer. Start by laying beneath the bus and watching/listening as a helper turns the wheel and makes the noise, maybe you can narrow down the source. It's very likely the steering box has internal issues, but inspect all the ball joints, tir rods, swing lever and drag link for binding, play and if they are catching on something. Look at the rubber disc between the steering column and steering box as well.