The only thing that comes to mind is a loose or worn suspension part? We usually charge a small fee to pinpoint the cause of noises like these, but we could discount the testing charge or credit the testing charge toward a repair. Did you have a wheel alignment service performed recently? Give us a call and set up a time to swing by. We would be glad to check it our for you. We might be able to give you a better idea if I could hear your description over the phone.