I've created a pull-request for an improvement to tonesweep synthesis: https://github.com/PaulStoffregen/Audio/pull/359

In short the frequency adjustment wasn't being applied per sample, but per-block, leading to spectral artifacts at
multiples of 344.5Hz from the carrier. Noticed when testing FFT visualization code.

Here's the FFT (1024 points) before and after the fix:
Click image for larger version. 

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The thin peak is a constant sine tone, the broader one is a tone sweep caught mid-sweep.