refrigerator noise

This queston is reguarding a 48" side-by-side GE Monogram refirgerator. Unit works great, however, about 3 weeks ago it started making a vibrating sound. Perhaps the best way to describe the sound is that of a jack hammer, of course not as loud. It makes this sound every 20-25 minutes and last for 4-5 seconds. Have tried to place my hand inside both the refrigerator and freezer compartments while it is making this noise, but cannot feel anything. Possibly due to insulation. I have not tried pulling the unit out away from the wall, and listen, as it is a built in unit and was difficult getting it in place when installed. Also don’t want it in the middle of the kitchen while trying to locate source of noise and then waiting for any part or parts that may have to be ordered. Any thoughts or ideas would be greatly appreciated.
Thank you for your time and help,
Stephen