personal experience: i bought my tamron 17-50 2.8. at first didnt realise anything was wrong, until i was shooting some stuff tat required long exposure, or against white background. realsied my photos had consistent specks that were very distracting. turns out after testing against a pure white bg, it was dust inside lens.
wad did i do? i sent to tamron service center (somewhere around changi? cant remember) and had it fixed. also got them to tighten my lens as the front is wobbly. now its as good as new.
dun need go exchange. send to servicing will be better. they can help u clean inside and recalibrate if needed.