My current bath fan generates a lot of noise and I figured that the motor inside is not tight to the frame. I want to replace the bath fan. From a brief research, I think I have to go up to attic and replace the fan from there. I measured the fan (7 1/2 x 9 1/4) but could not find same size of the fan. Should I cut ceiling and fill the gab with something? Please give me some idea how it is done normally.