Little late now to do anything.
http://www.soundproofingcompany.com/soundproofing-solutions/soundproof-a-ceiling/
http://www.soundproofingcompany.com/soundproofing-solutions/soundproof-a-ceiling/
Check out the link joe provided choose which option works best for your situation.I know I can not eliminate it but if I was able to get some batting up in there will it help or am I wasting effort?