Or, the motor in your fridge that runs the compressor could just be bad i.e. a brush has a slightly too large gap to the armature and is sparking. Or there is an inadequate or loose ground inside the fridge chassis.
If you know someone that has the same fridge as you do, try plugging their's into your rig in the same configuration as yours. If the noise goes away, its the fridge. If it stays, it is somewhere in the eletrical system of your truck.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang