Yes, Stephen is absolutely dead on about the idler pulley. My '98 Contour with the Zetec made that exact same sound, unfortunately three days into a long vacation trip...fortunately in Joplin, MO, where there was an excellent Ford dealer. Two pulleys, one timing belt, and $600 plus later, and the Contour was as good as new! The '98 was at about 107,000 miles on the odometer at that point, and I had just had it in for regular servicing and for a pre-long road trip checkup five days prior, because I felt it just wasn't right when I accelerated.... I was very lucky it worsened to the "rattling marble" stage you describe when I stopped for the day, rather than ending up in trouble out on the road. Sort of like the bearing whine in the engine compartment on the passenger side heard at the same RPM range in all five gears written off as "license plate bracket rattle" and "silly woman" by the first dealer, but corrected as a failing water pump by the second dealer. Get it in for service pronto! Judith