Apex was complaining earlier about fungus on his Tamron rear element.
Invest in one of these lens ring wrenches
http://www.ebay.co.uk/itm/3-Tips-DSL...item3f3a8d3589
and clean that fungus off before it munches through the lens coating.
(Tip: when dismantling use a small sticker on the outside of the element as a flipped element will cause chaos.)