It is funny how some people can make light of this by saying, avoid the bumps. Humor aside, it is really irritating. What you can probably do at the moment is to make sure all the bolts and nuts are tightly secured. Sometimes if the bolts that you are using are incompatible with the fender, you will end up with loose bolts and nuts, hence the rattling sound. You can also observe where the rattling is coming from. There could be other issues as well so it is better to inspect and address it. If you can’t fix it, I suggest you have it checked by a professional, they usually know how to fix these things.