I was sitting on the patio directly outside of my house on my 18 acre farm in west central Minnesota reading book at the picnic table. I heard a sound like air being displaced. The best way I can describe it is if you were to attach a ball to a rope and swing it, or maybe the sound you hear when your on a swingset, but milder than that. I was sitting facing south and I could hear it behind me and over my right shoulder (from the west / north west direction). I looked up and the aircraft was basically directly over head with a bearing of east / south east. I had my eyes on it for probably 20 seconds before it was over the horizon of the hill to the east side of my property.
It's ~70 degrees Fahrenheit outside and there is not a cloud in the sky, so I had an unobstructed view of it as it passed over. It appeared mostly dark brown in color, although there was some variation, and I remember thinking at the first second I saw it, wow that's a big hawk. But I immediately realized it was not as it was very obviously geometrically shaped, traveling at a high rate of speed with a perfectly straight trajectory and with no apparent movement in the craft. I confess to feeling bewildered as I sat watching it which apparently kept me from picking up the camera I had placed in front of me for photographing ducks in the pond, but I'm not sure I would have had time to pick it up and focus it even if I had the sense to do so at the time.
The main thin about it that threw me off was the bizarre sound, or lack of sound. It literally made a constant whooshing noise, which did not change in frequency as you would expect from something like the Doppler effect you hear from a car passing by, but was a constant volume and tone from the moment I heard it until it was over the horizon at which point the sound completely subsided. It as faint. If I had been listening to music at low volume I would have never looked up. It was roughly as loud as the surrounding birds, but obvious enough to take notice.
There was absolutely no noise that you would normally associate with an aircraft. No turbine noise, no propeller noise, nothing like that at all. There was only the faint whoosh as it traveled.
I can recall a few seconds into watching it, really when it flew over my grain silos (which are 70' tall) that I attempted to take survey of its altitude. I was and continue to have a really difficult time making sense of it's apparent altitude. I was bewildered at this while I walked from outside to the computer, and now that I've had a moment to reflect I think the reason I was having a hard time establishing perspective is due to the complete lack of noise or contrail, one of which has always accompanied aircraft I've viewed in the sky. I used to live near a marine base in NC where f-16's and other military craft flew by and the sound was always a good indicator for me of how high they were. The other thing I had a hard time reasoning through was how fast it was moving. I was thinking as it passed that it would have had to have been at low altitude to appear to be moving as fast as it was, and for a brief moment I questioned whether it was a remote controlled jet of some kind, but there are definitely no rc planes that look like that, that make no noise and are even close to that fast. Plus I live miles from my closes neighbor and they certainly aren't interested in rc planes. What I can say is that it had to be at least 4 times higher than my 70' silo, but beyond that I really can't say for sure. If it was at an altitude of say, 280', which now that I say it sounds very unlikely, I would guess it was traveling at least several hundred miles per hour. If it was at a higher altitude than that, and traversed the distance it appeared to, then it was really moving fast.
In terms of size, again, I'd be guessing due to my uncertainty on it's altitude, but the description I selected above is probably the case, that it would have been obstructed by a golf ball at arms length. If I was begged down to a guess I'd say maybe 30' - 50' in width and roughly that in length.
I'm at a loss attempting to describe why I can't provide a better explanation of either the features of the aircraft or it's altitude. The best way I can describe it is that as I was looking up at this thing, I just felt confused. It's not often that you look at something and flat out can't place it. In the brief time it was in front of me, my mind was focused on shuffling through first the possibilities as to what it was, and before coming to real resolution on that, it left my line of site while it passed over the silo at which point I started thinking about what altitude it must have been at and then my mind went back to the sound and I tried to focus on the aircraft for any sign of propulsion, and then it was gone.
Here's what I know for sure. I'm an engineer with a solid understanding of how the world around me works, I'm also interested in pretty much anything with an engine, including aircraft. I've been to the USAF museum at Wright Patterson as well as other aircraft museums including both NASA installations. I've seen a b2 up close and it wasn't that. Could it have been an f-117? Maybe??? Does an f-117 make zero noise whatsoever? I haven't seen one in flight, but I doubt it very much. Are they now painted brown? I don't think so. Aside from that comparison, which I feel would be a bit of a stretch, I've never seen and definitely never heard anything like it before. I sure would love to know what it was though.