Do you have aftermarket rims? If so, then I am pretty confident that your problem lies within those. The reason I say this is because it sounds to me like your wheels are not fitting on the hub properly. What I mean by that is if you remove the center cap from your wheels you will see the main center bore of the wheels, through that you will see an object that the center bore aligns itself onto and sits on, more or less. If the center bore of the wheels does not sit pretty flush on the hub, the object I spoke of in the last sentence, then your wheels can vibrate up and down on the hub at speeds. This is not good! This can lead to a very dangerous situation. Your wheel studs will control this to a degree but after awhile the stress will cause them to break and trust me you do not want that to happen. However, it is a common thing. Most people that buy aftermarket wheels do not know how to properly select correct wheels for their cars and so they will just get a wheel they like with the right bolt pattern and throw them on there. It is a little more complicated than that and in all honesty a lot of wheel places will sell you wheels that do not properly fit.
There is a remedy to this incorrectly fitting wheel situation though. The remedy is hubcentric rings. These rings mount over your hub, the thing that the center bore of the wheels slide onto. These things are usually made of metal or a hard plastic material and they will take up the gap, if there is any between your wheel and the mounting section of the hub and therefore stop the vibrations that you are describing as well as save your wheel studs and possibly more. This may not be your problem but I suggest you check it out. Just pop the centercap off and look to see if there is any gap between the wheel center bore and the mounting hub. If there is the smallest gap then you need hubcentric rings. Check that out and if there is a gap get back with me and I will explain to you how to measure this gap so you can get the correct size rings. Good luck!
