Why is there a constant hissing coming from bottom of fridge?

Vacuum leak. Open the hood and try to locate where the hissing sound is coming from. Likely a rotted rubber vacuum hose somewhere.

Could be water dripping on the heater when defrosting

I would suspect there is no refrigerant in the system any more. Sounds like there was a leak (hissing and odor). Not worth getting it fixed if that is the case.

Check to see if the cold air supply from the freezer to the fridge section is blocked and that the fan is running. The fridge uses cold air from the freezer to cool and many times the intake is blocked by something in the freezer compartment.

water inside, too much condensation from humid air entering fridge from bad seals. replace the seals. could also be that the condensate drain is clogged. if it's leaking from the bottom of the fridge, make sure the tubing is still inside the condensate pan underneath the fridge.

sounds like two problems,, but easy to cure,, first take the front plate of the bottom of fridge clean the dust from under the fridge and see if work right,, if not clean the line going up the back

Hi,Your husband did good job, after cleaning and vacuuming, did freezer (bottom) have excessive ice?, if there's excessive ice in bottom, after cleaning, you have a defrost problem, but if there're no ice in freezer after cleaning, you got freon (gas leak) , there's a condensate loop, that passes through ref. body, from compressor that naturally develop, freon leak after few years, by rusting, normally it can be by- pass. vacuum and refill with freon, if there's no freezing, but compressor running, you'll need a mechanic, you got a freon leak. good luck.

you hear a constant running water sound? look in your manual under "normal operating sounds". it explains all of the sounds. it sounds like you're hearing the refrigerant going thru your evap. coil located behind the rear frzr panel. you unit goes into the defrost cycle roughly every 8hrs and water drip down into the condensate pan un the refrig.

The only thing I can think of that you may be hearing would be if the compressor has a problem. If you notice cooling performance decrease or it stops cooling, I believe you will find the compressor has bad internal valves. It will probably take a technician to properly diagnose this and if the compressor has this problem the only answer is to replace the compressor.
I hope this helps, supertechks

It sounds like a defective ref section evaporator. This is common, due to poor quality metal used in fabrication. If you call subzero, they will more than likely fix it as they are aware of this issue. It probably wont cost you anything if you get them involved.