Refrigerator seems to run continuously (or very long times)
If it's "bubbling" while it's running it could indicate improper charge. A refrigerant system should have liquid at the metering device. If there is still vapour in the mixture(saturation temperature) then you are loosing capacity which would explain the long run times. It doesn't mean there is a leak it just means that it's not running at full capacity.

In this image the metering device is an expansion valve. www.wikipedia.com

It gets pretty complicated but to aid you in your search for the problem if something you try makes the bubbling sound go away you may be on the right track. Keep in mind any changes to system take about 15 minutes to stabilize. Sometimes just pulling the fridge away from the wall does the trick. With fridge has bubbling often indicates too much heat overall in the system.