A relative's new Civic has a rather low-pitched noise happening between high 30's-low 40's mph. Seems like some kind of rattle/vibration somewhere in front half of car. Doesn't get any worse on hills. Mainly during simple cruising without any particular load on engine. Two visits to dealer (including getting on lift to check heat shields) revealed nothing. Any ideas ??

 

Not saying it's the answer to your problem, but the Mrs' CR-v had a rumble/vibration cruising at speed. The dealer finaly force balanced the tires and found 3 of them to be out of limits. 3 new tires cured the problem.