I have a 2004 Sedan 5spd. At precisely 60 mph I have a slight vibration. But only at 60 (within a couple of mph). Every other speed no problem. It drives very smooth at every other speed. The wierd thing is it's not constant. It's somewhat rhythmic. Sort of a pulsating. It does it for about 2 sec then stops for about a sec. And keeps doing it over and over until I either increase or decrease my speed. I had a similar thing when I drove a Hyundai Elantra. Never had it looked at on the Elantra. Any thoughts?