If you're somewhat handy you may try removing and cleaning the throttle body. It's not too hard on the 850's. Spray brake cleaner works very well. Check the Volvospeed web site "Bay 13". There's a small vacuum line connected to the flame trap device that gets plugged with hard carbonlike material. You need to remove the plastic housing and drill the hole out. Check that your air filter doesn't need replaceing. Make sure all you vacuum lines are in good shape. The whistle could be caused by the ac recirc flapper not working. The actuating rods brake on the 850's. They're behind the glove compartment.