You miiiiiiiiight wanna give us more information. What’s it sound like? Plasticky or metal to metal? Where is it located? Inside or outside the car? Did it come on gradually or suddenly? What, in particular, were you doing to the car when the alleged rattle started? Hard shifting? Braking? Cornering? What was the road surface? What side/end of the car does the alleged rattle emanate from? How loud is it? Can you make it better/worse by changing any of the drive characteristics (I.e., cornering in a particular direction, braking, accelerating, etc)? Have you done any recent maintenance or upgrade work to it? How long have you owned the car before this started? Have you loaned it out to “that” relative recently? Did it change any of the performance metrics, like stalling, sluggish takeoff, hard or loose gear changes? Does it have curb feelers installed? Have you noticed any loose or missing parts anywhere on the car?
As you might guess, the more information you provide, the more concise our answers become. It’s kinda linked like that.