What are the characters in this seal? (Characters identified: 拏雲心事人不知)
the characters are “拏雲心事人不知”, read from top to bottom, right to left.
the “script” is slightly distorted from standard 
“拏” (to grasp, to reach) + “雲” (cloud) is a metaphor of “shoot for the moon” / “have lofty aspirations” (比喻志向遠大)
http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000056063
so, 拏雲心事人不知 would be:
the lofty aspirations (拏雲) in my heart (心事), nobody know (人不知)
it’s from the poem “去歲君逺㳺送仲梁出山” by 元好問, of 金 dynasty

去歲君遠㳺今年客它州青天萬古一明月只與行人生暮愁問君㳺何許情多地遐兮偏處處金鞭㫁折騏驥死萬里長鴻思一舉憶識子梁王臺清風入座無纎埃華嶽峯尖見秋凖金眸玉瓜不凡材西園日晴花滿烟五雲樓閣三山巔玉樹瑶林照春色青錢白璧買芳年・三年一夣南陽道・汴水迢迢入秋草・拏雲心事人不知 ・千首新詩怨枯槁・破屋仰見星踈衾風露清匣中有長劒爲君鳴不平泥途乆辱思一濯去去舉足皆清冷鄧州大師材望雄愛客不减竒章公軍中宴酣笳皷競銀燭吐熖如長虹幕中多士君又徃談咲巳覺南夷空東州春迴十月後梅華分香入春酒平生得意欽與京青眼高歌望君乆浙江南下青沄沄石門細路蒼烟屯五松平頭白日静千山萬山如亂雲菊源不逐時事改芝嶺自與啇顔鄰他日想思一廻首漁舟時問武陵人

or, derived from the poem “致酒行” by 李賀 of 唐 dynasty

零落棲遲一杯酒・主人奉觴客長夀・主父西逰困不歸・家人折斷門前栁・吾聞馬周昔作新豐客・天荒地老無人識・空將牋上兩行書・直犯龍顔請恩澤・我有迷魂招不得・雄雞一聲天下白・少年心事當拏雲・誰念幽寒坐嗚呃