My DC25 is developing an issue in the brush bar. If you hold the Dyson still but have it kicked back so the brush bar spins up you can hear it altering tone. Like its getting faster and slower, even although the Dyson is stationary. I have held it for a minute or so, not moving it and you can clearly hear the tone vary. Is it likely the brush bar motor is on the way out? Or could it be any other issues with the assembly?