i agree with PhillipSTD on this one. the rolling feed call was definitely meant to sound like multiple ducks simultaneously. to add a little flavor to it, i do it a couple of different ways. da-dut, da-dut, dut, dut, dut, da-dut, etc. i also like to add a little gorilla grunt to it for a raspier sound. i will use the ticka-ticka-ticka sound as well. it just took a lot more call time to learn. practice, practice, practice. you'll be surprised at how quickly you pick it up though. i went from sounding completely stupid with a call to sounding not bad in about 1 month. but that was discipling myself to blow it everytime i got in my truck to go somewhere and in my office (co-workers pick on me about it) whenever the opportunity presented itself. i was talking to a buddy of mine the other day and he asked how it was coming along. i blew it into the phone and he was shocked at the improvement and the quality of the sound..
just work at the rolling feed, eventually you'll hit a sequence and figure out how you did it. mostly though, it's just about training your tongue to get the proper timing. good luck.