Yep I agree with Mr P. Sounds like soething is moving in the drivetrain when you nail it. Such as spring bushing or mounting bolts loose or missing. If its an auto you may see if you can get it on a drive on lift and then power brake it a little being real careful. That way you can be under the car just like it is on the road and maybe see something moving a bit when you power brake it. But again I stress be very careful if you do that. Ron