Frigidaire refrigerator runs 24/7?

When it was first installed it made a very odd kind of thumping sound occasionally, but usually before it shut off. The other day it made the sound and stopped but did not restart as it usually would. I flipped the on/off switches then unplugged the unit but could not get it to run. A half hour or so later it started again and except for a few minutes the next morning it has run nonstop. It is quieter than it has ever been (9-12 months old) and has not made the sound since.

Is it common in newer models to run constantly? Even before this incident it seemed to run unusually long with an occasional short shut off.

I have only used refrigerators that were at least 20 years old and I know that commercial units run nonstop. Is this just the way they are now?

Thanks.