Clicking sound from cimex

[leofry@1966]

this has been gong on awhile not sure how long, the cimex still does the great job its always done since day one, but there is this annoying clicking sound. It does not always do it, and it stops sometimes during cleaning and you think man i am glad that stop but then a few minutes later it starts again. I first thought it might have been a little piece of a rock or something that feel between the guard and the motor but i don't thing that's it.

any ideas on what it could be ?