If it still rotates smooth, 99% of the time the BB's are worn. Absolutley must disassemble the motor, every so often. Use electrical contact spray to clean all, including BB's when cleaning. Make a note of the small shims on the armature shaft, when reinstalling them. So important. But clean and and lube the BB's before they explode and brick the armature. Or then you need a $new$ motor. Go with your gut, noisey worn BB's. Buy a new set, maybe even Ceramics. What I do when replacing any of my motor BB's. A BLX motor can last for numerous years as long as you keep up on the BB's. If you let them explode, plan on buying a motor often. A motor will be like new again with fresh BB's. But clean and lube them regularly. Every 15 runs or so, Your mileage may vary.
Always buy Motor Specific Abec 5 or above rated High RPM BB's. Use a drop of Synthetic High temp Oil on them,
even when new out the package. I use SuperLube and Liberty oils. Liberty (AMZ) is slightly thinner, less drag on the BB and Race. Depends on the motor I am running. Liberty oil actually tested better than many RC boutique oils out there at 5 times the price.
Jims, TRBRC and Avid make good motor specific BB's. Unlike the rest of the drivetrain, I feel Ceramic BB's
are well worth it with BLX Motors. Handle High temps well.
Technically, these RC Ceramic motor BB's are considered
Hybrid Ceramics. Because only the Balls are made of ceramic. 100% ceramic raced BB's are costly and not feasible or practical for RC use. Only Used in Commercial extreme applications. Some of these cost as much as $250. each.

Go to Boca Bearings and see for yourself.
>>$20.00 give or take is usually the going price for (2) Ceramic RC Motor BB's. Not tragic at all, considering the price of a new replacement motor. Larger motors, the larger BB's cost more obviously.