Whistling Sound

So I have a whistling sound that I get between 2700-4000 rpm. I went ahead and jacked up my car and crawled underneath it to locate the sound and found it to be coming outof my tailpipe. Could this be as simple as a hole inside the muffler? Or does this tell me something else is wrong?

Also when the car was jacked up and left in neutral with my parking brake off(5spd), I noticed the driveshaft was turning, along with my rear right wheel. IS this normal too?

Thanks
Alan

not sure about the noise without being there but the driveshaft turning is normal

Sound Gone. Put on a Flowmaster 50 SUV Series. $160 installed.