Electric Furnace / Heat Pump Question..

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We live in a manufactured home with an electrric furnace / heat pump installed. Whenever the furnace is running, it takes air from within the house an a little air from outside via electric controlled valve on an outside vent duct. It seems as though when the the furnace is running, it requires more air in than is coming out as we have noticed cold air being drawn into the house backwards through the Microwave vent, the bathroom exhaust vents and around door locks. I have managed to stop the incoming air around the door locks, but the rest has me baffeled? Is this normal? The furnace filters have been changed every month and the heat pump radiator within the unit is clean and free of dirt. Still it seems as though the furnace is starving for air to the point of where the sides of the furnace are drawn in when running and flex back to original shape when it isn't running.