If it's any consolation, my sister-in-law's relation had her 4 yr old car doing the same thing and we found the lug nuts coming off. The kicker, is that they'd been installed by a tire shop with an impact! It's become common to see a warning to re-tighten lug nuts after 100-150km up here. Everybody uses a torque wrench instead of the old, and true, arm-strong method. Crank 'em until they're tight- not insanely tight, just good, old fashioned tight and check 'em later- I bet they'll still be ok. Glad you found it!
