My 2005 Mazda 3 with 73K miles (roughly 20K on the tires) on it starts to make a sound, just like your washing machine does when all of the clothes go to one side and its still spinning, when I get to 50 MPH. When I increase to 60 MPH or higher, it not only makes the sound, but it starts to resonate. What is the problem?
UPDATE: I went to get the tires rotated and balanced, and they did that for me. And since then, the sounds I hear and vibration I feel have gone down ten-fold, but it is still there. Is there anything else that could cause this?
REFERENCE: For anyone else that comes later and wants to know where or what to look for when looking for their wheel weights (other than the ones Bob has already shown in his answer).