What was found
AIR RETURNED TO DEPARTURE DUE TO "RECIRCULATING SMOKE" NOTED ON EICAS AFTER TAKEOFF, APPROXIMATELY 4,000 FT. NO SECONDARY COCKPIT INDICATIONS. QRH DIRECTED CREW TO LAND AT NEAREST SUITABLE AIRPORT. F/A REPORTED ODOR, BURNED MID CABIN AROUND ROW 9. RECIRCULATION FANS, PERFORMED SMOKE AND OR ABNORMAL ODOR IN PASSENGER CABIN AND OR COCKPIT FIM 21-20-00-810-801-A. ISOLATED RT & LT RECIRCULATION FANS AS THE CAUSE. REACTIVATED LT AND RT RECIRCULATION FANS - PERFORMED FIM 21-20-00-810-801-A. COULD NOT DUPLICATE MESSAGE OR SMELL. FOUND MID AVIONICS 2 FAN HITTING. R & R MID AVIONICS FAN 2, REF AMM 21-26-04. ACFT POWERED FOR OVER 3 HOURS WITH NO MESSAGES OR ODOR.