FLT CREW OF B752 EXPERIENCE STUCK MIKE ON DEP FROM ORD.

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STUCK MIKE ON DEP. ON LIFTOFF ON RWY 22L ORD; WE NOTICED A SIDE TONE ON OUR RADIOS. I DID NOT HEAR ANYONE TALKING ON THE RADIOS; SO I CALLED ORD TWR. NO RESPONSE. I TRIED SEVERAL MORE TIMES AND THEN SWITCHED TO GUARD. ON THE FIRST FEW TRIES ON GUARD; I RECEIVED NO RESPONSE; SO WE SQUAWKED 7600. WE WERE LEVEL AT 5000 FT BY THIS TIME AND FLYING RWY HDG (LAST ASSIGNED). WE SUSPECTED WE HAD A HOT MIKE BECAUSE OF THE SIDE TONE. WE BEGAN A L TURN TOWARDS GIJ. DURING THE TURN; I FINALLY COULD HEAR ORD ON GUARD (OVER A LOUD SIDE TONE) GIVING US INSTRUCTIONS. WE EXECUTED THE HDG AND ALT CLRNC. WE WERE IN AND OUT OF COMS WITH ORD FOR ABOUT ANOTHER MIN OR SO UNTIL WE FINALLY ISOLATED THE PROB TO THE CAPT'S MIKE. AFTER PROB SOLVED; DISPATCH SENT A MESSAGE INDICATING WE WERE THE HOT MIKE. ONCE COMS WERE ESTABLISHED; WE CONTINUED TO IAD UNEVENTFULLY. DURING THE PROB; THE CAPT AND I VERBALLY DIVIDED OUR DUTIES -- PNF; PF. SUPPLEMENTAL INFO FROM ACN 616752: INTERMITTENT RADIO CONTACT ON 121.5. REACHED 5300 FT BUT RETURNED TO 5000 FT WHEN GOT TEMPORARY RADIO CONTACT TO MAINTAIN 5000 FT. FINALLY ISOLATED PROB TO THE CAPT'S RADIO. USED FO RADIO. MADE ACARS CONTACT WITH DISPATCH TO CONFIRM OPS NORMAL BUT 7600 SQUAWK COORD WITH DISPATCH TO FLY CLRED RTE IF ANYMORE RADIO PROBS PER FAR. COORD WITH LEAD FEDERAL AIR MARSHAL ON BOARD SINCE GOING TO IAD.