Try tracing upward and see if it is coming from one of the 2 rubber hoses leading to the front half of the airbox. Those are supposed to have plugs in them to keep out unfiltered air. Normally, the plugs are removed to drain out condensed oil vapour that came from the crankcase breather hose each time the bike is serviced. If the hose is big, (maybe 1/2 inch OD or bigger) it may be that it is the breather hose that is supposed to connect to the airbox.
