Fourier Synthesis of randomly generated Sinusoidal waves using Math.NET Numerics

I'm trying to create a generator (i.e. a IEnumerable<double>) that mimics the variability of a power grid frequency signal. I had the idea that if I overlay some noise over the top of a series of composed sine waves, I might be able to get the kind of plausible shape I need.

Here's a good example of the kind of curve I need to reproduce. Not necessarily with the frequency excursion, which I can handle separately.

Alas, my math is a bit shaky on this and I can't find any nice libraries to handle the composition itself. Does anyone know how to piece together a new distribution given a set of generators based on Generate.Sinusoidal (see here: Math.NET Numerics Generation)