What was found
FLT NR 434, MCO-BOS, AIR RETURN TO MCO, FUMES OF UNKNOWN ORIGIN REPORTED IN CABIN AFTER TAKE OFF. COMPLETED QRH FUMES DISSIPATED. COMPLETED SMOKE REMOVAL. QRH AND FUMES RETURNED WHEN PACKS RESELECTED ON RETURN TO PACKS OFF CONFIG AND FUMES DISSIPATED AGAIN. (AIR CONDITIONING PACKS-SINGLE) R&R NR 2 AIR CYCLE MACHINE IAW 21-51-05. OPS CHECK NORMAL, NO ODOR PRESENT.