IMC REPORTER THOUGHT ACFT IN CLOSE PROX ACCOUNT LOUD SOUND SIMILAR TO ACFT DEPARTING.

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WE WERE ON THE LOC AND G/S OF RWY 22R AT ORD; INSIDE OF SALTZ; AND CLRED FOR THE ORD RWY 22R ILS APCH. OUR ALT WAS ABOUT 2500' AND TURB WAS MODERATE. WE HEARD ANOTHER JET PWRED ACFT VERY LOUD; VERY CLOSE TO US FOR THE BETTER PART OF 30 SECS. I NOTIFIED ORD DISPATCH AND TALKED TO ORD TRACON AFTER WE LANDED. THE NOISE WAS THE SAME AS WE HEAR IN OUR AIRPLANE COCKPIT WHEN WE ARE AT THE HOLD LINE. THE NEXT AIRPLANE FOR TKOF; AND THE ACFT IN POS; APPLIES TKOF THRUST AND DEPARTS. THE ONLY DIFFERENCE WAS IN OUR CASE; THE LOUDNESS CONTINUED LONGER THAN WHEN WE LISTENED TO OUR ACFT TKOF FROM OUR COCKPIT. I HAVE BEEN A CREW MEMBER ON ACR FOR 18 YRS OUT OF MY 22 YRS WITH THE COMPANY. THE NOISE WE HEARD WAS DEFINITELY NOT FROM OUR OWN AIR CONDITIONING SYS OR ANY OTHER SOURCE INSIDE OUR ACFT. ORD TRACON INDICATED THAT NO C/A'S WERE GENERATED BY THEIR EQUIP. I FILED A NEAR MISS RPT WITH ORD TRACON IN ANOTHER TELEPHONE CONVERSATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: COMPANY AND UNION INVESTIGATED THE INCIDENT FURTHER AND WERE ADVISED A MIL ACFT; JET MLG; HAD DEPARTED NGU AND APPARENTLY PASSED IN THE VICINITY OF THE ARRIVING LGT. CONTENT AT NO TIME WERE THE ACFT CLOSER THAN 1500'.