I recently noticed my exhaust tone to be slightly louder. Almost as if I have a Y-Pipe. My car is stock. Do I have a hole in my muffler or something?

 

roll the windows up

 

tis possible you have an exhaust leak

 

Check all your bolts and joints between pipes... actually check for rust too... severe rusting can also cause leaks (in my experience, that is) It be more... but a leak is one of the easier things to check for...

 

Listen to your car's exhaust system from front to back. An exhaust leak is easily identifable by its aesmatic putt putt putt sound. Also keep in mind that colder weather makes your car 'seem' louder because of the characterists of metal.