sorry everyone, the lens I am having problems with is a Sigma 50-500mm lens. Dust has got inside it, from quite early on, and Sigma claimed it is because the lenses are not sealed (but warranty does not cover sorting it out), the coating is coming off it, and the thread end of the lens is working it's way loose from the rest of the lens - only a matter of time before the major part of the lens drops off if I keep on using it.

With dust as well, I'm a wildlife photographer so when the lens is in use it is outside, when not in use it is in a bag or case.