rmental

2005 EX Special Edition w/50k miles. Sitting in line at McD's with window down heard loud rattle coming from rear of vehicle. Checked out later at home...noise coming from body of muffler. Tailpipe shaking in time with rattle/noise. Checked under hood...engine has same vibration that cycles on/off with AC or slight rotation of steering wheel (PS pump load) or high alternator load (high blower with headlights on). Tach steady at mark (750rpm?) in park or in drive, AC on or off. Dealer says I need new muffler...I think the rattle is a by-product of the engine vibration. Run perfect except for the vibration at idle. Loud enough to be embarrasing on nice days with windows down. Any ideas where to begin?