Refrigerator smell

Came home to a strong smell eminating from behind my 15 year old Frigidaire. Temperatures in both the freezer and refrigerator are fine. Pulled the unit out, dusted and searched from problems, nothing apparent. Smell appears to be worst when the compressor is on. Any help is very much appreciated.

Here are your parts
Parts for Frigidaire FRT22TSEK3 Refrigerator - AppliancePartsPros.com

What kind of smell?
Where is the small outside the unit, in the fresh food compartment or in the freezer and where is it the strongest?