Patricio, thanks for the reply. I'm really not sure where the hissing is coming from. :/ I had to listen again when I got home last night. I let the machine run for a while to warm up, then when I turned it off, I could hear a sound coming from the lower coil area (I have the inner panel that covers the coil pulled back while I'm troubleshooting this) but I couldn't pinpoint the source exactly. It's not a leaking sound, BTW. I just mentioned it because it seems to indicate there is freon in the lines. I just wanted to be sure I wasn't misreading that signal, or that the sound is normal when a freezer is turned off. My main concern at this point is trying to confirm if there is freon in the system, or if I'm looking at another cause and from what I can see, it seems like there is freon.
Last night I pulled the defrost thermostat and put it in some ice water to do a resistance test on it and it failed that test, so I have ordered a new defrost thermostat. I also tested again the regular thermostat behind the knob and even bypassed it (even though it passed) but that still didn't help. Will a failed defrost thermostat allow the unit to run but not get cold? Can you bypass a defrost thermostat? I've read that you can and that you can't. I would assume bypassing it would be as simple as connecting the two wires that connect to it. Still, I'm not confident that this will fix my problems. A failed defrost thermostat will cause the freezer to be too cold, right?