B727 FLT CREW DESCENDS BELOW CLRED ALT DUE TO HEARBACK READBACK PROBLEM.

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I WAS THE FO ACTING AS PLT MONITORING. ON APCH TO GRAND FORKS (GFK) WE WERE INSTRUCTED TO DSND TO 3000 FT AND CONTACT APCH. GRAND FORKS APCH ACKNOWLEDGED OUR CHK-IN; ASKED FOR A PIREP AND GAVE US A CLOCK POS FOR THE FIELD TO RPT THE RWY FOR A VISUAL. I ASKED THAT HE CONTACT THE TWR AND ASK THEM TO TURN UP THE RWY LIGHTS TO HELP WITH SPOTTING THE RWY; 35L. THE GND WAS SNOW COVERED AND BLENDED WITH A DECK OF CLOUDS S OF THE FIELD THAT I DID NOT PREVIOUSLY NOTICE. WE WERE GIVEN A HDG OF 300 DEGS TO INTERCEPT THE LOC AND I HEARD AND READ BACK DSND TO 2000 FT. THE NEW ALT WAS PLACED IN THE ALT ALERT WINDOW AND ACKNOWLEDGED PER COMPANY FOM. IN THE DSCNT I XMITTED THE TOPS WERE AT 3000 FT AND THE BASE WAS AT 2400 FT; NO TURB OR ICING. AT APPROX 2200 FT WE WERE ASKED WHAT OUR ALT WAS AND I RPTED OUR ALT AND DSNDING TO 2000 FT. APCH STATED THE MVA FOR OUR POS WAS 2800 FT AND TO CLB TO THAT ALT. WE COMPLIED AND INTERCEPTED THE LOC FROM 2800 FT. WE SUBSEQUENTLY BROKE OUT OF THE DECK AT 2800 FT AND WERE CLRED FOR A VISUAL APCH TO RWY 35L AT GFK. THERE WAS NO INDICATION FROM ATC THAT THERE WAS ANY CONFLICT OR PROB; HOWEVER THE CAPT CONTACTED THE TWR LATER THAT DAY TO ASK IF THERE WERE ANY UNRESOLVED ISSUES; WHICH THERE WERE NONE.