What was found
AFTER T/O PASSING 235 KTS & CLIMBING THROUGH APPROX 4000 FT ACFT BEGAN TO VIB WITH LOUD NOISE COMING FROM BELOW & AFT OF PILOT SEATS. PACKS TURNED OFF, NO HELP. THROTTLES BACK TO IDLE, NO HELP. FLAPS DOWN TO 22 GEAR DOWN 210 KTS, NO HELP. GEAR UP 180 KTS, VIB WENT AWAY.