AN MD SUPER 80 IN CLB AT FL290 DIVERTED DUE TO EXCESSIVE VIBRATION ON THE L ENG CAUSED BY A COMPRESSOR BEARING MALFUNCTION.

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WE DEPARTED THE GATE AT XA04 WITH SHIP #XYZ. THE ACFT WEIGHED 135000 LBS; WITH 34500 LBS OF FUEL. WE HAD 70 PAX AND A CREW OF 5. WE USED A STANDARD PWR TKOF AND CLBED TO FL280 WITH A REQUEST FOR FL310. UPON ATC CLRNC TO CLB; WE USED THE AUTOTHROTTLES AND PERFORMANCE SYS TO CLB. ABOUT FL290 THE ACFT STARTED TO VIBRATE. AT FIRST; I THOUGHT IT WAS A PRESSURE PROB; BUT THE CABIN WAS FINE. WE THEN RECEIVED A CALL FROM THE AFT FLT ATTENDANT; TELLING US THAT IT WAS VERY LOUD IN THE AFT OF THE PLANE. THE ENG GAUGES WERE ALL NORMAL. WE THEN PULLED THE L ENG BACK TO AN IDLE AND ADVANCED THE R. THE NOISE AND VIBRATION DECREASED A LITTLE. WE THEN ADDED THE L AND RETARDED THE R ENG THROTTLE AND FELT THE VIBRATION WAS INCREASING. WITH THIS INFO; WE DECIDED THE L ENG WAS BAD. BY THEN WE ADVISED ATC THAT WE WERE RETURNING TO DFW AND HAD TURNED AND STARTED DSNDING FOR DFW. WE LANDED ON RWY 18R AT ABOUT XA56 WITH THE ACFT WEIGHING 128000 LBS. THE FIRE TRUCKS STAYED BEHIND US TO THE GATE. I DID NOT DECLARE AN EMER AS WE DID NOT SHUT DOWN AN ENG AND ALL ENG GAUGES WERE NORMAL ALL THE TIME. I WOULD LIKE TO KNOW WHAT WAS WRONG WITH THE PLANE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE VIBRATION WAS FROM THE L ENG AND A SECONDHAND RPT INDICATED THE CAUSE WAS A COMPRESSOR BEARING FAILURE. THE RPTR SAID IT RESULTED IN AN ENG CHANGE. THE RPTR SAID THE ENG WAS A P&W JT8D-217.