The Mercedes-Benz SLK is a two-seat roadster. It has a hardtop, convertible roof. There are a number of reasons why the roof can rattle. Depending on the source of the rattle, different approaches can be applied to fix it. A rattle usually indicates that something is loose. This could be the trim panel rattle against the components in the roof or the roof rattling against the car itself.
Drive with the top up and locate the source of the rattle. Have a friend drive or have them look for the source of the rattle. This job requires two people to do safely. Follow the source of the rattle with the ear. If it is coming from the front (where the roof meets the windshield), it is likely a connection issue. If it is coming from the body of the top, it is likely a body issue.
Examine the connectors on the windshield. Stop the top half way when coming up and examine the connectors on it. Are they damaged? A damaged connector on the roof can result in that part not being secure and thereby rattling. Check the connections on the windshield. If the clips where the roof locks into are damaged or obstructed, it could cause the roof to rattle against the windshield. Remove any obstructions.
Put the roof up fully. Check the trim lining the roof. Is the trim loose? Are some areas solid while others can be pushed in and out? If this is the case then the trim is loose from the roof structure and, as the car moves, the roof bangs against it. See whether any screws have fallen out. If they have, replace the screws and tighten the loose trim. If they haven't, the trim might need to be glued back to the roof structure.