Sorry Sid, can`t help with the rattle
The only thing that comes to mind, is there an electro-magnetic clutch by any chance, or possibly a vibration dampner mount has let go.
Separate story....
I had to replace our Frig-o-boat at about 22 years of age due to getting the compressor and controller wet-story posted here previously.
I picked up a new-never installed Alder Barbour built around 20 years ago, added an up-to-date controller and coreless fan motor and it ran for two seasons. When it quit cooling I determined from rereading the installation instructions that I had failed to torque the connector assemblies fully-lost the R-12 charge.
Pulled the system out of the boat and took it to a refrigeration shop. They determined the controller was fine, the compressor was good and suggested I reinsatall it on the boat and they would service it there.
They installed one or more inline `tees` to be able to drain and flush, used, I think it was R-490 refrigerant-closest cousin to R-12 meaning you don`t have to worry about evacuating every molecule of oil to replace it like you would for an R-134a conversion. He determined the system volume and it works flawlessly.
Not in keeping with `ìf it ain`t broke! I have picked up a sintered bronze thruhull exchanger and will have it cut in to the system to eniminate the fan and cooler.
Realize this is an extended thread-Just a comment about Franks`early troubles. When you`re on shore power the battery charger is topping up the batteries but the fridge draws off the batteries regardless, the compressor is going to see 13-14v instead of 12v plus or minus. These fridges never sees 110v AC To minimize line loss between the battery(s) and the compressor they recommend keeping the conductors as short as possible.