Farkas seems to have a mighty deep hand in all the cities guard force. Hes really edgy all the time too!

http://youtu.be/km30EIWqdx8 (my bad clip ps3 filmed on tv)

http://youtu.be/0yIJ7mBRq-Q (This guy had same issue.)

Thing is I don't have a bounty in Whiterun. Albeit there be a bounty elsewhere - as some say the bounty is the issue.
All I can say is.. we he isn't a guard!