How to generate a sinusoid with STFT at a frequency which is between bins
A sinusoid with a frequency that is between bins in the FFT frequency domain is circularly discontinuous in the time domain.  So you can't use the same IFFT results back-to-back without the noise from this discontinuity between each IFFT window, as the end of one window will have a value too far from the beginning of the next window (unless your frequency is bin centered, e.g. exactly integer periodic in the FFT's length).

If you calculate and vary the starting phase for each successive (complex input) IFFT, then you might be able to reduce or get rid of noise from the window discontinuity.

Note that the spectrum of a "between bin" frequency sinusoid is not just a cross-fade between the two nearest bins, but a Sinc shaped spectrum, spanning the full width of the FFT.  (Or a windowed Sinc function for a decent approximation.)