Simulink vs m.file

[cotdien]

Hi,
I generate a signal from Simulink, then I add a random noise to it by 2 ways:
- Always in Simulink with the block "random number"
- In workspace with the command "randn".
I keep the same SNR and identify the frequencies from the noisy data, but the effect of this noise to the signal is different. The data comes from Simulink is easy to identify, but not the data from mfile.

Any body know about that?

Thank very much.