Manual says use octane rating 87?

You list the very vague US as your location, so no one can help you with any accuracy, but here goes.

For the vast majority of the US, below 4500 feet in elevation, regular is 87 Octane. For some higher locations, regular is 85 Octane because the thinner air prevents 'ping' or detonation. It is possible you live at high altitude. If so, all is well. and you may continue to use 85 Octane.

If you are at sea level or plan to drive to sea level, you want at least 87 Octane.

 

A knocking sound during braking would not be due to insufficient octane, as combustion is not occurring during braking. It's likely something like warped brake rotors or rust buildup on the rotors.

In addition, as these engines have a knock sensor, you probably won't ever hear knock caused by too low octane, and the computers will respond to that appropriately to prevent engine damage.

That said, personally, I'd stick with 87 even at altitude, for two reasons - the first reason is that the variable valve timing system may reduce the effects of high altitude (and therefore take advantage of higher octanes than 85 at your altitude). The second is... if I were at high altitude, and planned to drive to a lower altitude, I wouldn't want to have to worry about what octane is in my tank, and whether it's appropriate for the altitude that I'm driving to.