rattling refrigerator when door closes

Hi, I have a 2 year old Kenmore (model 970-678722) refrigerator with an odd problem. When I close the door, often I hear a rattling sound. It starts louder and slowly subsides...lasts up to 20 seconds. Seems to occur more at night for some reason, possibly when the house is warmer?
I have observed the sound when simply pulling on the door, so it's related to the change in air pressure. Even with the fridge unplugged, the sound persists, although it seems that after a few attempts it no longer occurs when unplugged.
I am stumped. The sound originates from the bottom right of the fridge. I've leveled and raised the front as per the manual, and vaccumed everything to no avail. The base warrantee is up, but the compressor and sealed unit are still covered. Would love to know what I'm getting into before calling for service.
Any thoughts? fsm2k2 at yahoo dot com
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Could be copper coil just touching the wrong spot. I also wonder whether these newer refrigerators have a valve to let out air when you close the door. Expensive models used to have that feature. If it does maybe that is the source of your noise. Also if you have an in the door icemaker that may let the excess air out. Check that to see if it is the source of noise. There may be ice collecting on the flap in humid warm weather so that would explain the temperture affect.

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