I've implemented an FIR filter on the BF518 EzBoard to filter the audio and send it back out, both via the SSM2603 codec (24-bit in & out at 96kSa/s). The filter looks good, except I find that I have a spectral artifact around 3*f (18 kHz), where f is the input frequency in the passband (6 kHz). I'm at a loss as to what is causing this. I've tried bit shifting the accumulator output to make sure I was catching an overflow, but that only dropped the output amplitude of the whole signal. When I shift the input signal up to 3*f, it is effectively suppressed. Attached is a capture of the output (blue trace) with an overlay of the spectrum of the input signal. Is there something simple I'm missing?