interpolation - Real-valued ringing when zero-padding odd-length FFT

So I'm trying to write a frequency-domain interpolator that zero-pads the frequency response of a signal and inverse transforms. There's two cases I have to deal with:

1. Even-length response - have to split the $F_s/2$ bin because it's ambiguous. So I copy the negative part of the spectrum, and add n*(interp-1)-1 zeros in between.


2. Odd-length response - there is no $F_s/2$ bin so just split positive/negative frequency and insert n*(interp-1) zeros between them.

The code that does the zero-padding can be seen here

// Copy negative frequency components to end of buffer and zero out middle
//  inp    - input buffer of complex floats
//    n    - transform size
//  interp - interpolation amount
void zero_pad_freq(cfloat_t *inp, size_t n, size_t interp) {
    if ((n % 2) == 0) {
        memmove(inp + n*interp - n/2, inp + n/2,     n/2*sizeof(cfloat_t));
        memset (inp + n/2 + 1, 0,       (n*(interp-1)-1)*sizeof(cfloat_t)); // Duplicate Fs/2 so we need one less zero

        inp[n/2]          /= 2.0;

        inp[n*interp-n/2] /= 2.0;
    } else {
        memmove(inp + n*interp - n/2, inp + (n+1)/2, n/2*sizeof(cfloat_t));
        memset (inp + (n+1)/2, 0,         (n*(interp-1))*sizeof(cfloat_t));
    }
}

The first case is working fine, I'm testing it on a chirp signal and it interpolates just fine, there's a little numeric noise, but it's round tripped through an FFT so what can you do (first $50 \mu s$ or so of the signal show):

The problem is with the odd-length transform, I'm getting a pretty heinous transient response on the real samples only ($50 \mu s$ again, real):

The imaginary channel has a small ripple on it, but not nearly as bad:

It's like I've screwed up my $F_s/2$ bin in the odd case, but there is no $F_s/2$ bin, so I'm very puzzled. Anyone have any thoughts?