After several years, we began experiencing the noisy brush beater bar noise. In our case, we cleaned the bar of all strings. hair, fibers etc. that were wrapped around the bar, but this wasn't the cause. Our belt was fine as well. The culprit turned out to be a roller bearing that is located on a spring arm and keeps the belt tensioned. Years of grime, hair, fibers, etc. had worked its way to the axle of this bearing and had wound itself around each end finally building up to a point where it spread the forks of the bearing retainer enough for it to dislodge. Luckily it stayed in the brush housing where we found it rattling around.
It took awhile to figure out how to get the brush housing out of the cleaner head assembly see here:
Then, we: a) took off the motor cover, b) cleaned out the bearing retainer forks, c) cleaned off the bearing axle and d) replaced the bearing. We reassembled the unit and now it works like new. I suspect many people having the DC17 beater brush noise problem actually had the same problem but perhaps their tension bearing got lost or was sucked up into the cyclone.