It's the room of requirement. Voldemort required a room to hide his item in. Where better to hide a small, reasonably nondescript item than in a pile of junk? If not then it's possible when he hid it it wasn't full of junk; Harry required a room full of all of the small, nondescript items that may have been the diadem. Yes he knew what it was but maybe not specifically enough to single it out.

The room also has a habit of overdoing things, like when Dumbledore needs the loo and finds tens of chamberpots there, when one would do: 


  "Only this morning, for instance, I took a wrong turning on the way to the bathroom and found myself in a beautifully proportioned room I have never seen before, containing a really rather magnificent collection of chamber pots. When I went back to investigate more closely, I discovered that the room had vanished. But I must keep an eye out for it. Possibly it is only accessible at five-thirty in the morning. Or it may only appear at the quarter moon - or when the seeker has an exceptionally full bladder." 


~Christmas Ball, Goblet of Fire,

Why did Voldemort assume that no-one knew about Room of Hidden Things?
About two days ago I got my tires rotated on my 2005 Chevy Cobalt while I was getting an oil change. This was the first time that the tires had been rotated in at least 14,000 miles (since I purchased the car). The car itself has about 36,000 miles, so I'd assume that's the age of the tires.

Anyways, whenever I go above 30 mph I hear a helicopter like sound, and it ramps up the faster I go.

Here's two videos I've recorded:


http://youtu.be/bb0gZGLJJ5M  
http://youtu.be/Ol6MMqPVfgQ


I've read that it can be caused by waiting too long to get the tires rotated, as they start to wear into a pattern, which can cause the noise.
