I was entering the house though the back door and heard a noise like a helicopter but louder and more of a chop chop than the normal helicopter noise.I looked in the direction it was coming from and was taken by complete surprise at what i was looking at .I lived for a long time around a military base and have also done military time so i knew what i was looking at was not in any classification of a man made aircraft.First of all it was about the size of a school bus only maybe a little smaller than a full size bus .It was rectangular in shape and had no discernible windows . there were protrusions more like squares from the front to the back but to be honest i could not really say what the design was . i instantly thought of getting the camera but i knew the way it was moving i had no time to locate and get the camera up to running to catch a picture. i observed it go straight west to east crossing a road at about i estimate 300 feet from the ground and probably 500 feet from me .I turned but not at an aircraft would with a banking motion.It simply rotated from left to right about 15 to 20 degrees as it crossed the road .There were no exhaust ,no wings ,no tail , no windows that i could see and no way of possibly staying in the air since there was no propellers ,no engines ,no balloons ,no heat signature of any kind and the only noise i heard in the beginning i can't remember if it was still present as it was moving .To me it appeared as if was searching for something the way it moved from straight ahead to the right ,after this movement it completely changed direction from going west to east after crossing the road to the direction of east to north.There were clouds in the north direction and it headed upwards and at one point it tilted from the left side upwards and i could see the top of the ufo and it looked very much like a flat slightly yellow rectangular surface ,again , no visible anything to suggest it had a power system of any kind .It went into the clouds at about a quarter mile from where it changed direction.I lost sight of it at this point and quickly cussed at myself for not having the time to get a picture of the probably only time in my life to see a ufo or what ever it was as close as i had experienced .I did not report this as it would have had to have been observed by people on the road i know and at the time of day that it was there was a 99% chance no one would have seen it as quickly as it happened .I don't know where it came from because i only saw it as it was almost directly north of where i was standing and watching after i saw it . the next day i heard this same chop chop noise but where i was in the house i could not tell in what direction it was coming from and by the time i got to the back door i no longer heard the noise. i have not heard this noise since.I do hear helicopters going over the house and also checking on the power lines .And i can honestly say they are not the same noise as this craft was making .It was a lot more pronounced noise .I was thinking it was a good impression of a helicopter but not 100% the same noise .The thing that struck me the most was what ever i was looking at should no be able to be in the air .You simply cannot have a large rectangular anything and have it move in the air and change directions with no observable power system of any kind . i don't know what the ends looked like because it never moved in a way that i could see the ends ,the only way i could tell it was rectangular was when it moved into the clouds so that i could see the top it have a very definite rectangular shape .I could estimate it was probably 5 feet by 20 feet but since it was a quarter mile from me at that point of estimation i could be way off .I have seen other strange things but never as close as this one was.