2003 Land Rover Discovery Exhaust Howling Noises

My Rover makes a howling/wooing noise after it's been driving for like 15 or 20 minutes. When the car is in drive and your stopped, you can really hear it. Not sure if it's a exhaust valve, air injection issue or a hole in the exhaust.

Again, the vehicle has to be warmed up and when you're idling in drive, you hear it. Putting it in park, not so much because there is less strain on the engine. You hear it idle at about 600 rpm and when driving again about 2000 rpm. Please advise