It could be as simple as a collapsed baffle inside of the muffler itself. Are you having any other problems with the vehicle? Is there a loss of power or rough running. If so there is the possibility of a partially clogged catalytic converter. If not, I would wait until the vehicle is cold, and bang on the muffler to see if there is anything rattling around in it. If the muffler is looking rusty, or if you hear any rattling, I would just go ahead and replace it. I have seen instances where a performance muffler like a glasspack for example is installed backwards and the exhaust makes a whistle or squeal because the baffles catch the exhaust flow and cause resonance. Check it out and let me know if I can be of anymore help.