Diamond shaped object flying south from Seattle towards Portland with 1 light on each corner and a red beam rotating from underneath.

I was 15 and was staying the night at a girl friends house along with another friend.  Her parents and grandparents were playing bridge in the dining room. One of my friends had her car there and we were going to go to the store but her battery was dead.  Another friend down the road about 3 blocks had jumper cables so we were walking down to get them, when one friend pointed to the tree tops and said "What is that?"  It looked like 3 head lights coming over the trees. Before I had time to think she yelled "oh my god" and we all started running back to the house.  When we go to the porch I looked out from under the over hang and saw a large diamond shaped craft flying just above tree level.  There was a bright headlight colored light on each corner with a red light in the middle that had a red beam shining out and rotating around.  It was grayish in color with no apparent seams.  The sound was similar to an electric razor hum but not as loud.  It flew about 5 mph in a straight line with no sudden movements or turns.  We watched it until it disappeared from sight.  My best guess is that it was about twice the size of a football field.  We try to get my friends parents to come and look but only her little sister came out. When she looked up she let out a blood curdling scream and ran in the house. My friend refused to talk about it and I have not seen them since that year.
Date is approximate.  PD