FLC OF MLG DSNDED FROM ASSIGNED DSCNT ALT IN RESPONSE TO A TCASII RA AND MOMENTARILY DECLARED AN EMER.

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ON APCH INTO ONTARIO WE WERE CLRED FOR VISUAL APCH FOLLOWING AN (ACR Y) PLANE WHICH WAS IN SIGHT. I DECLINED THE CLRNC AS WE WERE ABOUT TO ENTER BROKEN LAYER OF CLOUDS AT 12000 FT MSL. ABOUT 30 SECONDS AFTER ENTERING CLOUDS; OUR TCASII GAVE US A TA FOLLOWED BY A DSND COMMAND RA. THE TCASII SCREEN SHOWED TFC IN FRONT AND BEHIND US AND WE INITIATED A DSCNT. ATC FREQ WAS CONGESTED AND ABOUT 15 SECONDS PASSED BEFORE ABLE TO XMIT. DUE TO CONTINUED RA I DECLARED AN EMER; HOWEVER ATC WAS UNABLE TO UNDERSTAND DUE TO TCASII NOISE LEVEL. I WAS UNABLE TO COMMUNICATE WITH CAPT; ALSO BECAUSE OF THE LOUDNESS OF TCASII ALERT. WE THEN BROKE OUT OF THE CLOUDS; THE ALERT CEASED; AND WE SAW THE ARPT AND ASKED FOR A VISUAL APCH. WE WERE CLRED FOR A VISUAL APCH TO 26R AND CANCELED OUR ATTEMPT FOR DECLARING AN EMER. WHEN WE TALKED TO ATC ON THE PHONE AFTERWARDS; SHE STATED THAT TCASII ALERTS WERE 'NORMAL' FOR THAT SECTOR. IN MY OPINION TFC CONGESTION AND RADIO CONGESTION IS A PROB IN THE ONTARIO AREA. THE LOUDNESS OF OUR TCASII ALERT ADDED TO THE CONFUSION.