How to find out the noise level of a signal
I just don't think you're thinking this through all the way, and using the toolbox in MATLAB before you understand the basics. SNR is signal-to-noise ratio and is defined as $\text{SNR}=\frac{\text{Signal Power}}{\text{Noise Power}}$. When you call snr(signal, noise) in MATLAB, all it is doing is calculating the signal power, mean(abs(signal).^2), and the noise power, mean(abs(noise).^2), taking the ratio of them and returning the result in decibels (dB). 

In the comments you are asking about how to calculate the SNR of x (the noiseless signal)...well its dividing by zero because the noise power in a noiseless signal is zero. You'd be doing $\frac{\text{Signal Power}}{0}=\infty$, you can go ahead and try using the snr function by doing snr(signal, zeros(size(signal))) but I'm willing to bet it either returns Inf or gives an error.

Note: Read the MATLAB docs on snr(signal) and you'll realize it is probably not returning what you expect. Docs are here: https://uk.mathworks.com/help/signal/ref/snr.html