pulsed humming sound, object zips overhead

I know this was a long time ago...but for the record. It was the late spring of 1971. I remember it was June as school was just about out for the summer. Not sure of the exact date but it was a Saturday morning. I and two of my friends would always take our dogs out for early morning walks to let them chase rabbits. They never caught any but had a helluva time at it. We were walking three abreast in a southeasterly direction. I was on the right side of the trio. I remember my friend Rick, who was walking in the middle was talking about something, when I heard this extremely high pitched, pulsed humming sound. It was really weird, not loud at all. The way I've described it to the few I've told this too is that it was kind of like a dog whistle in audible range. For reasons that I do not know I stopped in my tracks and looked up. I saw this "object" out of the corner of eye to my right, out over the hills of the southern peninsula in the bay area. I was now looking west-southwest. This thing zipped, and I mean ZIPPED overhead in a somewhat northeasterly direction, disappearing over the foothills to the east in a matter of seconds. With it went the strange pulsing hum. Suddenly, I heard Rick say, "What are you guys looking at?". I then proceeded to look forward to where Rick was now standing about ten or so feet further down the trail. Then I looked to my left and my eyes met those of my other friend Craig, who had also stopped in his tracks adjacent to me.. I could tell by the look on his face that he had seen it too! We then proceeded to tell Rick what we had seen. He did not see it but said he remembered the weird sound. The object was circular, kinda dark but was moving so friggen fast little detail could be seen. It left no trail of any kind. The sky was crystal clear and this sighting (and hearing) was very real. It wasn't swamp gas or a meteor, and it sure as hell was not a plane nor jet aircraft. It WAS an Unidentified Flying Object, period. Had never seen anything like it prior, nor have I seen, or heard, anything like it since.