What's wrong with my SUM logic?

I am trying desperately to replace the SUM call in my program but don't get the right answer. Any ideas?

Logic that works:
noise = SUM(diffSeries, Period)[0];

Logic that does not work:
noise = diffSeries[0] + noise - diffSeries[Period];

The program has an if statement ahead of the 'noise' statement that ensures CurrentBar is greater than Period. That's not the issue. I just get a different (larger) value for noise with my logic without using the SUM routine.

Thanks for the help!
G