My lens is back to life after it was down for some months. David has gifted hands!
Thanks so much for your help!
Thanks Octarine. I learned a lot of things from you.
Additional information on this lens:
The problem is common for 17-50mm irregardless of mount.
Even the new Nikon mount with built-in motor has the same problem.
No intention to scare users. Tamron 17-50mm F2.8 is a value for money lens.
But if you notice a loose zoom, stop using and send immediately for servicing.
It's a simple tightening of screw. And with a good thread lock glue, no worries for a long time