The noise ordinance is at the bottom of this webpage:
Town of Cary Film/Video/Photography Policy (http://www.townofcary.org/Departments/Administration/pio/Town_of_Cary_Film_Video_Photography_Policy.htm - broken link)
I think individuals can work in their lawns on Sunday from 9AM to 9PM if the dB level is below a certain threshold. "No construction work is allowed on Sundays"...anyway- I'll let you read it!
What you did is best though- ask them to stop. (and thank goodness they did!) Too many people around here are so fast to just call police instead of talking with their neighbors.