Thanks Octarine. I learned a lot of things from you.My lens is back to life after it was down for some months. David has gifted hands!
Thanks so much for your help!
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