Noise carries. When you are in an airplane, there is a loud noise when the plane is taking off. It almost sounds like the pilots are revving the engines. Since the people are in the plane, they are so close to the noise, they would obviously hear it. Other people will hear the noise too.
However, it depends upon how close the people on the ground are to the airplane. There would be some noise but it would most likely not be as great as if they were on the plane itself. The further away the people are, the quieter the noise would be. Also, if there is an obstacle between the people and the plane, it would also shield the noise.