Furnace making "crackling" sound (and other stuff)

I have an older 2011 177 which has been in my possession since March 2011.  Yes, this is a sound that is heard every now and then, probably due to the metal expanding/contracting when heating and cooling and lasts for maybe 15 seconds or so.  How long has it been since being used and do you hear this when using both electric and propane or only one of those?

If one stove works and the other doesn't it likely points to a pinched gas line.  I believe the 178 stovetop is located in the slide portion.  Try lighting it when the slide is closed vs. open.  If it lights one way but not the other it is definitely a pinched line and should be relatively easy to find.  IMHO this is the likely scenario since the outside stove lights.  Do you hear or can you smell the propane when attempting to light the inside stove?  If not, then you line is pinched or you have a lot of crud built up inside each burner, with the latter being quite unlikely.   However, if inside stove doesn't light and you can smell gas, (when slide is in or out) then remove the line going from your propane regulator to the front of your 178.  If a black gooey substance drips out - that is where the problem lies.  Clean out that line and if bad get underneath and follow the propane line to the stove inside and open that line as well.  Hopefully you or someone you know has a compressor - blow the line out from the stove to the front of the pod and you should be good.  How is everything else working as this will also let you know if the dreaded black goo is in the lines or not.  Does the HW tank fire up and give a nice blue flame when heating? Also look for a healthy blue flame in the combustion area at the rear of the fridge when running on propane. If the lines had goo in them and have been cleaned out and the flames are yellowish when burning you will likely need to replace the regulator as well, which usually runs around $ 20 - $ 25.