When I drive it on the highway at higway speeds, its much more pronounced and louder when I downshift to 3rd gear. 4th gear is a little quieter and 5th gear its the most quiet but I can still hear it. It seems to get much louder when the rpm's rise and much more noticeable when I'm at 1/2 to full throttle as well. Not sure about the marbles in a can sound though. It's more of a metallic buzzing type of sound that seems like its coming from under the hood right in front of the firewall.