Vibration at any speed? Off and ON
 

I just bought my 3rd Miata and I have been getting a vibration at any speed. The vibration only lasts for about 5-10 secs and really doesn't affect the steering wheel. So that being said it also comes about 5-10% of my driving around. It does feel like it is shaking more from side to side then from up and down. I have gotten my alignment done and everything is spot on. I did try to yank on each of the front wheel with no noticeable amount of play(more for the popping noise the steering makes when making sharp turns/not this issue). I have not got to play with the rears to see what play yet. So what could you guys think it could be and has anyone else had a similar problem? I have read plenty of other threads on Vibration but not as off and on as what mine is doing.

If there's slop in your steering linkage (said pop) then the front wheel could be fluttering like a castor on a shopping cart. Do you feel it while cornering?

No like I said it doesn't feel from the front end, the steering wheel doesn't shake

Check the driveshaft, particularly the u joints.