A CRJ100 CREW FAILED TO RAISE THE ACFT'S LNDG GEAR AFTER TKOF FROM A BUSY ARPT BUT RECOGNIZED THE ERROR WHILE RETURNING TO LAND FOR THE EXCESS NOISE.

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AFTER TKOF AND DURING THE INITIAL CLB; A LOW RUMBLING NOISE WAS COMING FROM THE NOSE AREA. IT SOUNDED AS IF A SVC DOOR WAS OPEN. IT WASN'T A DOOR; THE LNDG GEAR WAS NOT RAISED AFTER TKOF. AN INTERESTING CHAIN OF EVENTS FOLLOWS. OUR DEP WAS FROM RWY 22 AT CMI. THERE WERE MANY UNIVERSITY OF ILLINOIS TRAINING ACFT IN THE PATTERN FOR RWY 22 DOING TOUCH-AND-GOES; LOW APCHS; ETC. THEIR PATTERN IS BUSY AND THE SPACING IS TIGHT. HOLDING SHORT OF RWY 22 WE WERE GIVEN A NEW ALT. WE WERE THEN GIVEN POS AND HOLD. THERE WAS A PIPER ARROW THAT HAD JUST LANDED AND WAS DOING A TOUCH-AND-GO. ON HIS CLBOUT HE TURNED L TO STAY IN THE PATTERN. AT THE START OF HIS TURN WE RECEIVED OUR TKOF CLRNC WITH A R TURN TO 360 DEGS. THE ACFT WAS LIGHT AND ACCELERATED RAPIDLY. AFTER LIFTOFF THE FO SAID CLR R AND AT 400 FT I STARTED THE TURN WITH A VERY HIGH DECK ANGLE TO HOLD V2 +10 KTS. IN THE TURN THE NOISE WAS NOTICEABLE AND AS THE AIRSPD BUILT UP THE NOISE AND RUMBLE INCREASED. NO KIDDING; FLAP RETRACTION WAS NORMAL AND WE LEVELED AT 5000 FT QUICKLY. WITH THE 'ODD' SOUND WE STAYED BELOW 200 KTS TO TROUBLESHOOT. I KNEW WE COULD NOT CONTINUE WITH AN OPEN DOOR SO WE REQUESTED TO RETURN TO CMI. I ASKED FOR FLAPS 8 DEGS AND 20 DEGS THEN GEAR DOWN. THE FO'S RESPONSE WAS THAT THE GEAR WAS ALREADY DOWN. I CHKED MY SPD; LET OFF SOME OFF COLOR LANGUAGE AND ASKED FOR GEAR UP. AFTER TELLING EVERYONE; DEP; FLT ATTENDANT; OPS; ETC; THAT THE PROB WAS SOLVED WE CONTINUED TO CVG. I THINK THE CONTRIBUTING FACTORS FALL INTO THE TYPICAL CATEGORY. WE STARTED THE DAY EARLY IN OMH. I WAS TIRED. CMI IS A BUSY LITTLE ARPT; LOTS OF DISTRS; EASY TO GET OUT OF YOUR 'GROOVE.' SOME TASK OVERLOADING AND MISSING STANDARD CALLS. A QUICK CALL FROM TWR TO CONTACT DEP; LOOKING FOR AND CONCERNED ABOUT TFC; ALL THIS ADDED UP AND I MISSED THE OBVIOUS.