My 08 has developed a roar under power, the engine is strong and the trans, works well, Had put new tires and thought that might be it but its not , when road surface changes the pitch in sound does not. Its a steady drone that increases and decreases with acceleration and deceleration . Thoughts ?

 

Welcome- it could be a broken header or damaged exhaust system. Try jacking it up, supporting the Z on jack stands and using a flashlight to inspect the exhaust system on both sides where it comes into the cats and y-pipe.