Sounds like a a venting problem. Most cars up until now days and some still do use vacum assist to change venting from hot to cold and changing different vent configurations. So you have a vacuum leek somewhere in the system. If they are mechanical then its a fuse most likely . also in older cars there were cable system's witch controlled everything. If you here hissing sounds you know you have a vacuum leek somewhere in the system. If there is a worrying sound something is jammed up mechanical problem. It's a needle in the hay stack sometimes trying to figure out and you have to do it by deduction to figure out what the real problem is. Good luck