Thoughts on vibration

So I'm getting a vibration that comes in about the time I hit 3rd gear in the 1/4 is there for about apprx 400 rpm then is gone till i cross the finish and comes back breifly then is gone,so i feel it is speed related not rpm.It was just kinda weird its not really bad,like shake your eyes out, but there.Was waiting for it to get worse as i went faster then it was gone untill decel and only there for a breif time probably as it slowed to that speed again.
Looking for your thoughts on this,not use to one that comes in then goes away.
Thanks FABO