Have you checked out examples and functions in the Signal Processing Toolbox, or spectrogram,fft,pwelch?
Broadband noise in the spectrum will be difficult to separate from the true signal unless you know something about each, like the signal is known to occur only at low frequencies, or in a certain narrow spectral range or something like that. Or else you just want to erase all high frequencies regardless of whether they're noise or signal.
Other kinds of noise, like interference from power lines, may occur as spikes in the spectrum, like precisely at 60 Hz or 50 Hz. Those can be detected and erased.
