Man. Diff value: WBC 7.2, RBC 4.6, PLT 256, neutrophil 37, lymphocytes 47 with atypical lymphocytes at 10. Mono test negative. What could be going on?

Context... A CBC without context (meaning symptoms, why it was done) is not particularly useful and nearly NEVER diagnostic for anything. Without knowing anything about your health when you had this blood drawn, I would say that the atypical lymphocytes are indicative of a viral illness and would not be concerned unless you have symptoms that are not improving and that you find worrisome.
Your test. Looks pretty normal except for the presence of the atypical lymphocytes. This can be caused by a large variety of things from hypersensitivity reactions to a whole host of infections both viral and non as well as some rare cancers. Your negative mono test is just a start in your evaluation. Your illness history along with your physical exam findings can direct your doctor to further evaluation.
What r your symptoms. The WBC and platelet counts are normal. You do not report Hgb or Hct, but assuming that your RBC are normal sized, you are not anemic. Atypical lymphocytes can indicate mononucleosis, but depending on the experience of the lab technician can be factitious. It would help to know what symptoms you have. Most 37 year olds have already had EBV infection (positive monospot test when acutely infected)