i bought the canon new 15-85mm lense last month, and recently i just notice something different on the zoom barrel, i don't know if i can explain this in a technical term,

if i hold my 7d , at flat level, or even 45 degree with lense end downward, the zoom stays, and i can zoom in and zoom out by turning the lense barrel with little effort

but if i were to hold the canon vertically down to shoot, and without holding the barrel by hand, the zoom drops down slowly (extend/zoom in) around 50mm to 70mm range, like the lense extends down by pulling gravity, feels like the friction on the lense barrel not strong enough to hold the lense from still.

do i need any adjustment?

the 17-55mm doesn't do this.