After running for over 9 years, 7/7 and 24/24, my Helios KWL EC 500 Pro started to make more and more noise. It was especially noticeable at night, even at a lower speed. Clearly, the motors were the culprits, and since they are brushless motors, the ball bearings were for sure at the end of their lives. I removed one of the motors, read the reference number, and found a very nice video on YouTube explaining how to change the ball bearings on a similar model. The reference of the motors for my KWL EC 500 Pro is R3G160-AC50-12. The ball bearings are the same as in the YouTube video. I ordered 4x SKF 6000-2RSH/C3 from brw.ch and after a few hours of work, my KWL is running silently like on the very first day.