Q: Car has developed a knocking sound

My car has 6,000 miles on it and the engine has recently started making a knocking sound. I can it inside the car, and it is very noticeable outside the car as well. I first heard it a few days ago when my son was pulling the car into the garage, and I was standing outside the car. It makes a knocking sound, which reminded me of how a diesel engine sounds. The engine runs smoothly and sounds fine, except when idling. I took it to the dealership, and they heard the noise and didn't know what it was. I left the car there, and they called me the next day and said they have Nissan techline involved. I haven't heard back from them yet. Any ideas what this could be from?