In reply to I'd rather be climbing:
I think if it was sand/grit then you would be able to zoom past it with a crunch. Sounds like something metal or hard plastic in the gearing. I persist with focussing helicoids until the grit moves somewhere else inside the lens and anything that slips past the sealing is only going to rattle around until it sticks to some grease anyway and not be a problem.
Taking it apart will probably leave you with an expensive paper weight and perhaps a shock at how badly made the lens is, depending on the price range of course.
If you don't want to send it in, just use it as you say and the problem might go away on its own.