Remove background noise from audio file (python or matlab)
Since you're adding white noise, the highpass and lowpass filtering will almost  not remove the noise in the frequency band where you want to keep your signal, so you will always have some background noise with this highpass and lowpass filtering strategy.

Not sure if this helps, it depends on the signal-to-noise ratio:
If you can clearly distinguish the noise from the signal in the spectrum (something similar as in the second figure of the Noisy Signal example in Matlab's documentation of the fft), you could set a threshold and make the spectrum with an amplitude below that threshold equal to zero before taking an inverse Fourier transform to get back to the (denoised) time-domain signal.

By the way, GuassianNoise = np.random.rand(len(FourierTransformation)) produces uniformly distributed noise. To get Gaussian noise, you need randn instead of rand.