Probably strut mounts or some bushing going bad. Relatively cheap fix but kind of a pain if you're doing it yourself. Also, try to listen to where exactly the noise is coming from. It might just be something loose in your trunk.
A forum community dedicated to Mazda MX-6 owners and enthusiasts. Come join the discussion about restorations, performance, upgrades, modifications, classifieds, troubleshooting, maintenance, and more!