Sigma sells parts to independent repairmen, so it should be easy to have it fixed for a decent price.
I repaired 3 different Sigma lenses during the last months, and I've been equally satisfied by Sigma's official service center, and by a local "approved" laboratory.
The advantage of using the "official" one lies in the fact that any lens they service is firmware updated. Sometimes that makes a huge difference...
From my own personal experience:
recent Sigma zooms are great, but they are way more fragile than they look!
A not-so-strong knock can bend the zooming and/or focusing sleeve, even if the exterior of the lens shows no visible damage.
There could be something loose in your zoom, but it's also possible that the part needs to be replaced.
In both cases the cost should be reasonable, cause it should be a quick repair.
I wish you good luck
Modern high cost Sigma zooms are just great, and from the way they look and feel you'd think they are well made, but I'm starting to believe that the Tamrons are actually sturdier.
I'm curious about the opinion of other Pentax users
Cheers
P