AN SMT HELI HAD NMAC WITH A C172 IN VICINITY OF UNCTLED ARPT. RPTR ACFT HAD BROADCAST ON UNICOM POS; ETC; BUT INTRUDER C172 MADE NO CALL OR ANSWER. BOTH ACFT TOOK EVASIVE ACTION.

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WE WERE ENRTE TO A SUBURBAN CORPORATE CAMPUS FOR A PAX PICKUP. OUR RTE TOOK US OVERHEAD 06C (SHAUMBURG ARPT) FROM SSE TO NNW AT 1700 FT MSL. WE CALLED OUR INTENTIONS AT A POINT 3 NM SSE ON THE CTAF (123.00). NO TFC WAS OBSERVED VISUALLY OR ON THE TCASII. WE CROSSED MIDFIELD AND 1 NM N OF THE ARPT; CALLED CLR BUT REMAINED ON FREQ. AT THE SAME TIME A TCASII TARGET APPEARED 3 MI AWAY AT OUR 11:30. WE WERE ABLE TO ACQUIRE THE TFC VISUALLY -- A LIGHT CESSNA SINGLE. AS THE ACFT APPEARED TO BE NO CONFLICT; WE CONTINUED ON COURSE; AND I (PNF) BEGAN A DSCNT CHK IN PREPARATION FOR OUR LNDG. WHEN I GLANCED BACK OUTSIDE (NOW 2 NM NNW OF 06C) I OBSERVED A RED/WHITE CESSNA SEL (PROBABLY A C172) INITIATING A SUDDEN L CLBING TURN. THE ACFT WAS APPROX 50 FT HIGHER AND ABOUT 100 FT AWAY AT OUR 1 O'CLOCK POS. THE PF INITIATED EVASIVE ACTION BUT THE CESSNA'S TURN PROVIDED CLRNC SUFFICIENT TO AVOID A COLLISION. AT THAT TIME WE HEARD OVER THE RADIO A CALL OF 'DID YOU SEE HIM?' FOLLOWED BY 'YEAH.' THERE WAS NO TCASII INDICATION OF THE ACFT; SO I SUSPECT HIS XPONDER WAS NOT OPERATING. APPARENTLY THIS ACFT AND THE ONE SEEN EARLIER WERE TOGETHER; ALTHOUGH THEY WERE NOT; BY ANY STRETCH OF THE IMAGINATION; FLYING IN FORMATION. A FEW SECONDS LATER; I KEYED THE MIKE AND SAID 'YOU GUYS COULD BE A LITTLE BIT TIMELIER WITH YOUR RADIO CALLS;' BUT THERE WAS NO RESPONSE. THEY CONTINUED ON THEIR WAY S AND DID NOT LAND AT 06C.