I had a problem, when the van was stationary, engine at idle, turning the steering either caused pump noise or the engine to struggle(can't remember which). Revving the engine before turning the wheel resolved the problem. Fault was the alternator free wheel pulley. I had the alternator replaced and problem was resolved. Not sure if this is what you've got, or why this solved it. Garage came up with this solution, cost under £200 total.