458 Rattle from left rear when hot

Pretty good rattle over bumps seems to develop on my new-to-me 458 Spider when it's been driven for a while in traffic on warm days and I hit some bumpy roads. Sounds like rear of the car, maybe left side. Stays quiet until then. Also quiets right down after it cools off some. Does not rattle on smooth highway, etc. At first I thought it was the license plate frame rattling but that seems fine. Any likely suspects?