Please read this answer all the way through there is something that may be relevant (strike out 'important') at the end !
Maybe it's this
"clip that holds the filter in place and the locating lug of the access flap to gently rub together as the ball rotates and create a slight clicking noise as the appliance is moved around (though is not routinely noticed due to the motor noise)"
(from Fully Cleaned Serviced Refurbished Dyson DC25 Animal Ball Vacuum Cleaner
stop press - I just saw this - if you do not have proper Dyson brand HEPA filters - see end of answer - you may get a loud clicking sound....
The manual tells you about removal and cleaning HEPA filter - the All floors version has one as well)
Otherwise I can only refer to Dyson support, there are various ways of contacting them given at the top right of this webpage Support forDC25 Multi Floor
(the user guide manual is downloadable from this webpage)
Sounds to me as if maybe there is an irregularity on the ball that is contacting the body/ball housing every time the ball does round....unfortunately this makes the ball diameter ~8cm which too small.
Beware Some After Market DC25 Filters Do Not Fit Correctly March 2014