HELP!!! Bought a used fridge because the one we had was too small. Guy said he noticed the freezer wasn't as cool as it once was but the ice didn't melt and everything seemed to be working great. Got it home vacuumed the coils...not Coils. Checked all the fans, the settings, the control board....find out we need need a start relay order the part, comes in we install it....not cooling. Decide as a last ditch effort to add freon....negative 15 add freon starts to cool like a champ....for about 2 hours the coldest it got was 50 degrees....then back up to 60....checked the freon again negative 20......now makes a continual hissing noise even when unplugged please help!!!