What was found
DURING CLIMBOUT WHILE ACCELERATING TO 300 KTS IN CLEAN CONFIGURATION, THE AIRCRAFT BEGAN A MEDIUM FREQUENCY VIBRATION. VISUAL CHECK OF WINGS SHOWED LEFT INBOARD AILERON VIBRATING UP AND DOWN PLUS OR MINUS 2 INCHES AT THE TRAILING EDGE, RIGHT AILERON PLUS OR MINUS 1 INCH. SLOWED AIRCRAFT TO 250 KTS AND VIBRATION STOPPED. AIRCRAFT RETURNED TO MSP AND LANDED WITHOUT INCIDENT. REPLACED WORN BUSHING IN THE LEFT INBOARD AILERON QUADRANT, RIG CHECK OK.