Hissing sound

I hear the hissing sound on my BMW 528i (1998). When I start the car I don't hear anything, then when I put into drive I can hear the hissing (like a bad bearing or so, originaly I thought it might be a water pump) sound coming from the front of the engine(where belts are). When I turn wheels to right and left sound gets stronger. When I put it in neutral or parking it's almost undetectable. When I press on the accelerator and start moving, I don't hear anything and when I stop at the signal I can hear it again. I went to the automechanic I told him to check the water pump. They said that threre were no leaks. I sat in my car and started it and suddenly tempereature shoot up in red. They checked it again and said that there was air pocket inside of the engine and they let it go through some vent and it should be fine and they started hearing those noises that I was talking about and they came up with conclusion that it was tensioner and it needs to be replaced. Well, I was kind of suspicious. Since then only one time temperature gauge went near red. I turned off the car and restarted it and temperature came back to normal. Since then it was fine, but the hissing sound is still there. Could it be blown gasket or something like it? So let me recap: hissing sound from front right side where alternator belt is and all other belts, sound increases when I turn wheels right or left, it disapears when in parking and neutral and when driving no sound at all. It does sound like some bearings might be bad(for alternator or tensioner or water pump maybe), but why it acts differently when in parking or neutral (belts are still rotating when in parking or neutral or driving) and suddenly starts when turning wheels or put in drive when I'm not moving (my foot is on brake). Maybe steering pump (it does leak a steering fluid just a little bit, not much)? I can't think of anything else, maybe head gasket?