[bestgenman]

I believe you are experiencing the defrost cycle. I’ll be very elementary here and hope not to offend anyone!   In heat mode you are really air conditioning the outside air. 

 If you think about it, when air conditioning absorbs the heat in the air, the result feels cold and the outside coil expels the heat.  A heat pump is a reverse cycle air conditioner.  Just reverse the coil functions and you have the hot air being expelled inside and the cold expelled outside.  The outside coil condenses water out of the outdoor air and it freezes on the coil.  A temp sensor imbedded in the coil triggers the refrigerant gases to go the other way.  So, the hot gases now go outside to defrost the frozen coil and the inside fan is turned off so it is not blowing cold air inside.  The swooshing sound is the reversing of the gasses.

Elementary, but you get the idea of how it works and what you are hearing.

Heat pumps are more efficient than electric heat until the outside temps start to get ~38 then their efficiency drops to the same as electric.  I run mine down to about 40 then switch over to aquahot.