My v50 seems to have an issue with the rear of the car. Above 25mph, the car gets considerably louder to where highway speeds it sounds like it has no exhaust. After a brief look, the exhaust seems ok. Could this be a stuck brake rotor or something else? I thought it was the exhaust,
but after you lift, the car seems to automatically slow. It seems to far back to be the transmission.

 

You might have a bad rear wheel bearing, they can get very noisy.

Jack the rear wheels up and spin each wheel by hand - when really bad they can be easily heard.

 

Sometimes even if they're not too noisy spinning unloaded, they'll still exhibit significant play when you try to move them by grabbing the wheel at 3 and 9 o'clock, and at 12 and 6 o'clock.

IIRC, swapping out a rear bearing isn't a really huge job (you can buy the hub and bearing already assembled and just bolt it in).

 

another of the tells of a bad wheel bearing is when the noise changes when cornering (such as a 40 mph sweeping turn that loads one side of the car more than the other).