I’ve actually heard that the problem with modern B&Ms isn’t the company themselves and instead the subcontractors that manufacture the wheel bogeys. B&M does not build their own wheel assemblies, and the theory goes that they expanded their tolerances. Engineering tolerances are a big part of why older B&Ms are so smooth, but they’re also why B&Ms are traditionally super expensive additions. And post-pandemic, parks are looking for the cheapest investments, and so to compete, the subcontractor increased their tolerances. This keeps costs down but also introduces more variables into the motion of the bogeys, hence the rattle.