Could it be the power steering?

On some cars the power steering is the power brake assist. The way to tell is if the master cylinder for the brakes has a large vacuum line coming from a large drum behind the master sylinder running to the intake. If it does then it is vacuum assist if it does not you will find hoses running ffrom the power steering pump to the assist module behind the master cylinder. When you turn or use the brakes, if they are power steeering assist, this puts a load on the power steering pump and can slip the belt on the pulley. Check the belt tension and fluid level for the power steering and if it is loose tighten it up and also top off fluid with what your owners manual reccomends. If it still does it you may need a new pump or at least a belt.