2003 Other Nissan Models banging noise

HI,my car recently began making a "banging" noise under the drivers side front end,and I can feel it thru the floor,it gets louder and faster with driving (acceleration) my brakes feel "spongy",however there are no problems with stearing,pulling ect. My brakes do not make any type of grinding noise,ect. I have an appt to take to the dealer for repair,any ideas what it is and what cost might be?? thanks so much Steph