I was driving to work today on a totally flat road and out of nowhere heard a strange clatter like a small part shook loose and shortly after the whole car started to shake like crazy. It seems like it's a rear end thing but I'm not sure -- it just gets more violent the faster I go and is really low pitched. No other sound is heard like a bearing whine or anything. No apparent loss of power or hard clutch either.
The engine itself doesn't do anything out of the ordinary if I rev it under no load, but when I crawl under the car I can't find anything loose, missing, or out of the ordinary at all! I still have no idea what the clatter was. At first I thought it was a ballast coming off of a rim because I noticed one was missing, but I went to get the wheels balanced (ended up buying all new tires) and there was no change.
Any ideas?