After going over a small bump in the road today , a rattle began to resonate from underneath the car (seemingly below the front seats). It could be easily heard and a noticeable vibration could be felt through the passenger floor mat. It almost sounded like something rubbing on the road, after stopping and looking underneath the car we realized that wasn't the issue. As the rpms climbed, so would the nose. It eventually went away, it came and went three or four more times after that. I'm not sure if it's an exhaust issue or something worse. The car is a 2009 Infiniti g37x sedan, auto, no sport package, no mods. I'd really appreciate any help, i'm at a loss and pretty worried about the car. Thank you in advance.