A CRJ700 SHORTLY AFTER TKOF EXPERIENCED A LOUD NOISE FROM THE NOSE OF THE ACFT. RETURNED TO THE FIELD. FOUND GND PWR DOOR OPEN.

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SHORTLY AFTER TKOF; I HEARD A VERY LOUD SIREN TYPE SOUND COMING FROM THE NOSE OF ACFT INCREASING IN PITCH AS SPD INCREASED. AT THIS POINT; I SUSPECTED A DOOR PROB. WHICH ONE? I COULD NOT PINPOINT PRECISELY. THAT NOISE WAS NOT ONE I HAD BEFORE (COM HATCH LEFT OPEN). BECAUSE OF THE RISK INVOLVING A DOOR DEPARTING (POSSIBLY DAMAGING THE ENG) AND DISTURBANCE OF THE RVSM CRITICAL AREA; I ELECTED TO RETURN TO ZZZ. ATC WAS CONTACTED BUT AN EMER WAS NOT DECLARED. I FELT SAFETY COULD BE GREATLY COMPROMISED IF THE SIT WAS NOT DEALT WITH PROMPTLY; BUT AT THAT TIME THE AIRPLANE RESPONDED NORMALLY. THE FLT ATTENDANTS WERE BRIEFED; COMPANY WAS MADE AWARE OF THE PROB. AN OVERWT LNDG WAS PERFORMED AT 70200 LBS WITH A RATE OF DSCNT OF 100 FPM ON RWY 25R (OVER 10000 FT LONG). UPON RETURN TO THE GATE; MAINT FOUND THE GPU HATCH HAD NOT BEEN CLOSED. THE OVERWT LNDG AND THE OPEN HATCH WERE ENTERED IN THE MEL. MAINT TOOK CARE OF THEM. I ALSO; AFTER THE EVENT; CONTACTED DISPATCH IN ORDER TO OBTAIN A NEW RELEASE. AT THIS POINT; THE DISPATCHER WAS UPSET BECAUSE I DID NOT CONTACT HIM DIRECTLY DURING THE EVENT. HE TOLD ME SINCE I HAD CHANGED MY DEST HE HAD TO SEND ME A NEW RELEASE TO MAKE SURE I HAD ENOUGH FUEL TO REACH THE NEW DEST (I WAS ON TOP OF MY DEST WITH ENOUGH FUEL TO REACH AN ARPT OVER 1200 MI AWAY). DURING THE EVENT; COMPANY OPS WAS CONTACTED. IN TURN; OPS CONTACTED DISPATCH. CONTACTING DISPATCH DURING HIGH WORKLOAD VIA TELESIS OR ACARS TAKES ONE PLT WITH HEAD DOWN; IN THE AREA WITH A TCASII HALF BLUE AND THE REST YELLOW AND RED IS IN MY OPINION NOT SAFE AT ALL.