I was on duty at base.
Saw, and heard craft.About 50' over building.
Sound changed when moving, from low to high pitch.
The craft was about 50' over the base dental clinic.
It was 4:00 a.m. and I was the only one on duty at the time.
It had 3 large oval translucent areas, in the craft, about 15' across and they were spaced about 30' apart.
There was a blinking light at either end about 20' above the ovals and about 15' out.
The overall shape could not be determined because of weather conditions.
I saw and heard the craft for 15 minutes before it moved slowly away.