Trippy planes with green lights

8 or 9 planes just flew overhead with only a flashing green light on them. I didn't get a good view of the planes just the green lights. They were all in a straight line moving south.

They were moving very fast, they moved out of sight in just 15 seconds. From horizon to horizon, only 4 were in view at a time. When one would go out of sight the next one would be in view. Really trippy.

The sound of the planes could only be heard when they were close. When the last one past and was in the distance, the sound didn't fade away but kinda just turned off. really weird.

Wonder what they were?