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My heat pumps are acting up, the living room unit runs but does not create any hot air and the rear unit has started making weird noises.  A couple nights ago it got down to 34 degrees F and I was running the rear unit with the fan set to run continuously at low speed.  After running fine for a number of hours the fan stopped but what sounded to me like the compressor kept running although it didn't sound quite right.  after about 3.5 minutes of this there was a loud air release sound, the compressor continued to run for another minute or so then the fan came back on heat once again flowed.  I tried to attach a audio file but it didn't work.
Not knowing the particulars of your heat pump unit I’ll take a stab at it. My previous Travel Supreme had a Dometic Penguin AC/Heat Pump. Once the temperature outside hits 39 it would make laboring sounds and not function. They’re  not able to work under 40 degrees I believe. So, once outside temp drops I’d turn off heat pump and turn on aqua hot furnace heat. 

Just my 2 cents.
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.
I appreciate the responses, unfortunately I'm experiencing an issue with the Aquahot and can't really use it to heat the house. I have a couple space heaters that keep it comfortable enough. I just can't run them both at once without heavily taxing the inverter.
Hi Jack and Dianna,

Since no two Newell's are alike my thoughts may be useless to you, but here goes anyway.

I am going to assume you are plugged in to 50A power. If you are boondocking with no AquaHot and it's cold, well good luck!

I am able, should I wish, to operate two 1500 watt portable heaters without any inverter use or breaker tripping when plugged into 50A. I found it very useful to know from where each 120VAC outlet in the coach is fed. In other words, what breaker turns each outlet on and off and the rating of the breaker. Good to know what other devices are fed from that breaker. When you understand how power is distributed in your coach you are in a much better position to manage that power yourself. My inverter bypasses when not inverting. In other words, any 120 VAC load on the inverter comes from outside post power, not inverted power make from my house batteries. Then, I also labeled each outlet in the coach as fed from the inverter or not. That way I could plug a heater into an inverter outlet and another into a non-inverter outlet with no issues at all.

I didn't think about the inverter passing the 120 through. The way I read you're comment is when on shore/generator power all your outlets work, some on inverter and some (those controlled by the breaker box} on pass through. If that is the case, are some of your outlets dead when fully running of battery power, or do they switch to inverted power? I'm pretty sure all of mine are hot all the time. If they are all hot regardless of power source how will tripping the breakers tell you which ones are inverted and which are pass through? Forgive me if these are stupid questions but electrical circuitry has never been my forte.
on my 02, when i am running just on inverter power (no shore or genny), only some of the outlets are powered. there should be a breaker box next or near your inverter that will tell you what is powered by the inverter or at least give you a starting point. with a single inverter, it is typically the 120v water pump, the 120v air compressor, the tv's and av gear, microwave, fridge and various outlets of the choice of the owner when it was built. if you have two inverters, could be an air conditioner and additional outlets etc.

I bought a Klien ET300 digital circuit breaker finder, and the accessory kit that can check light bulb sockets and 240v. It was a lifesaver when I remodeled our kitchen. I plan to use it to make a detailed 120v wiring diagram for my coach when I get it back.