Very low flying triangle with lights on the corners, large purple flash in the center, two witnesses

My long time best friend and I were 13 and 14 at the time.  We used to walk around the city late at night in the summer months...or go out into the woods...essentially, we were trying to sneak around the city without anyone seeing us.  We were on our way home one night in July, and through the trees that lined the street, we saw somthing that scared us both to death, and that we did not talk about untill just recently.  Through the trees, we saw three bright white lights in the shape of a very large triangle.  It was easily 150 long and at least 50 feet wide.  It was floating very slowly, heading west.  There was a noise coming from the craft...a very low humming sound.  It was almost undetectable - as a matter of fact, if we wouldn't have been looking right at the craft, we probably wouldn't have noticed any noise at all.  It traveled slowly, maybe 10 miles an hour.  Neither one of us could believe our eyes, and we continued to chase it all the way down the street.  The most incredible thing about the craft was how low it was flying.  At most, it was 50 feet off the ground.  The white lights on the corners of the craft did not illuminate the craft so much as the street, so we couldn't really see what color the craft was, or what kind of material it was made of.  As we reached the end of the street, we were almost directly underneath the craft.  It then started to bank to it's left (south, tward the woods).  It traveled a bit further, at the same pace, and then we lost view of it because of the trees.  We could still see the white lights through the leaves, and then, abruptley, their was a huge flash of purple light from the center of the white lights (and if I remember correctly, the humming got more irritating...the pitch didn't change, it just got more intense), and then it was gone.  Neither one of us saw any of the lights lift up into the sky, they were just gone after the explosion of purple light.  I also remember a few seconds after it dissapeared, when the two of us were virtually shouting at ! each oth er "Did you see that?", I heard a boom - or a loud quick rumble in the sky - but very far away...in the distance.  He and I both ran to his house, which was only a few houses down from where we were watching, and told our parents what had happened.  They didn't believe us, or thought we were imagining it.  We didn't talk about it after that night until just recently, but both of our stories are still the same.  It was a terrifying and incredible experiance.
Date is approximate.  PD