I was riding passenger traveling east on Jack London blvd with my father and uncle. The sky was completely clear, the temps stayed in the upper 50s all day as we have been experiencing an artic chill, and I was just staring out the window as I often do from the passenger seat. I noticed what appeared to be a very short jet contrail at a high altitude that would disappear quickly and reappear in puffs or pulses as the object, which was very hard to see, seemed to be descending because the contrail was vertical, not horizontal as it would normally be. The contrail was far more visible to the naked eye, and I only saw a very brief split second reflection of light off a solid object at the front of the trail. I had never seen short bursts of contrail like that before and asked, "what is that?" My uncle, who was driving, asked what I was looking at, not being one to take his eyes off the road. I described it to him and he said it was probably a pulse jet. Though I had never heard of a pulse jet, it seemed plausible and I left it at that. I continued to watch as disappear, never taking my eyes off it. Considering the object was nearly invisible and most of all I witnessed was the trail, I assumed it was a jet descending and as it did, less contrail was visible until I could no longer see any thing. It really did just seem to vanish. The whole sighting lasted no longer than 3 min. How ever some research in to pulse jet engines revealed that there has been little to no success with using them for air craft, except in the hypothetical case of the Aurora Project. While I believe what I saw may have been some sort of jet, it seemed a tad bit mysterious and maybe worth reporting.