White light moving very quickly low on the horizon with military helicopter following.

My husband and I were watching tv when we heard a helicopter.  Within a minute or so it got very loud and the windows started to rattle.  It was obviously a military helicopter flying low.  We have experienced this on occasion, but never have the windows rattled like this instance.
My husband and I looked at each other, jumped up and ran outside, looking north.  It was a clear night and an almost full moon, which was in the eastern sky.  What made this encounter interesting enough to report was not just the helicopter.  
We are somewhat in the flight path of PDX and have planes flying over, so I know how fast they go.  When we went out, the helicopter was just flying over our house in a NNW direction.  In front of it, also flying NNW, was a small bright white light -- low on the horizon and flying VERY fast.  No sound could be heard other than the helicopter.
Interestingly, there were two airplanes flying south higher then the light and the helicopter.  They had typical airplane lights and followed the normal flight pattern we usually see from planes flying over.
Again, what was noteworthy about this experience was the speed and low altitude at which the white light was traveling, plus the military helicopter following it (I don't know if I would say pursuing it). 
I don't know if the light was ET, but we would definitely call it a UFO because of the nature of the encounter.  Maybe something military?