I had almost the same problem several years ago. Turns out that one of the nuts on the wheel came loose. That should be easy to check.
Damn, I totaly forgot that when I put one of my winter wheels on I was missing a lug nut so one of my rear wheels only has 4. Totally forgot about that. I was on my way to buy one that day and got side tracked then forgot all about it. Hope thats it. Thanks