DIAGNOSIS That's a classic sign of a worn or damaged counterweight. Inside the saw, there's a counterbalance mechanism that moves opposite the blade to reduce vibration. At high speeds, if a bearing is shot or the weight is hitting something, it turns into a grinding shake-fest.
FIX This isn't a simple field repair. The gear housing needs to be opened up. The issue is usually with the needle bearings that the counterweight shaft rides on, or the shaft itself. You're looking at replacing the crankcase assembly (which includes the counterweight) or at minimum the specific bearings. For most people, this is a job for a Makita service center.
IF THAT DIDN'T WORK If the internal parts look okay, the problem could be external. A severely unbalanced or bent blade can cause insane vibration at high RPMs. Try a brand-new, high-quality blade from a different manufacturer to rule that out first—it's the easiest check.
HEADS UP Running the saw with that kind of vibration isn't just annoying; it's damaging other internal components like the motor bearings and electronics. If you hear the grind, stop using it on high speed until it's fixed.