Do you have alloy wheels? If se, are the wheel balance weights hooked on the outside of the rim vertcially, or stuck horizontally on the inside 'dish' where you can't see them?
I had a similar problem some time ago that I couldn't pin down. Turned out that I had a shitty wheel balance a the place I bought my tyres; went to another shop, got the wheels rebalanced and problem fixed. FWIW, they put the weights on the outside of the rims in the usual way, and explained that you can't get as good a balance with the weights stuck on the inside (which is what the first place had done)