i was putting some new brakes in my z last week. once i was done i spun my wheels while it was on the stand and theres something rolling around in my back tires. is this a common problem should i be worried what is it rocks or rubber

 

My tire also has something in it....I cant imagine what it could be

 

No need to be alarmed;They all have it.I think it's rubber if I remember correctly.I asked the local mech. awhile back about it . He said no worrys....................