98 blazer heater problems?

I have a 98 blazer with some sort of a heater problem I think. It makes a weird noise kind of like a sqeaking sound. The heater blows hot air even if the air is on. It will sqeak, blow hot air, stop
sqeaking and blow cold air. It just keeps doing this over and over again. Any ideas? I havent checked anything because Im not sure what to check. Any help would be appreciated. Thanks in advance.
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The HVAC system has a main blend door that controls temp by diverting air over the heater core/AC evaporator or both for mixed temp settings. This blend door is actuated by vacuum. There are many vacuium hoses and pots involved with the system.
So, it sounds like the vacuum source is marginal (maybe a damaged line) or you have a leaky vacuum pot that isn't holding the vacuum to keep the door open.
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