The influence of the surface and interfaces of semiconductor-metal Al/n+Si-nSi/Al and Ag/n+Si-nSi/Ag structures with different aluminum and silver contact coating layers on the level and behavior of the low frequency current noise spectra was experimentally investigated. It is shown that the level of low frequency noise strongly depends on the material and form of the contact coating layers. At the room temperature, at the frequency 10 Hz, the noise level for Al/n+Si-nSi/Al structures is equal to ~ 10-15 A2/Hz, for the Ag/n+Si-nSi/Ag structures is equal to ~ 10-17-10-18 A2/Hz. It seems possible that the interface conditions modification, which in its turn mirrors at the processes of surface reflection and refraction of electrons and phonons, affects on the relaxation processes of longer-wavelength electron distribution function fluctuation and thus on mobility fluctuation and 1/f noise level as well. On the base of noise spectral characteristics of the mentioned structures the peculiarities of the acoustic phonons refraction on the semiconductor-metal flat interface are experimentally investigated. Several practical aspects related with so-called "refraction points" are discussed. It is proposed that, by manipulation of those phonons "refraction points" at hetero-interface, it will be possible to suppress the part of 1/f noise level, which arises in the volume of semiconductor. It is supposed that the hetero-interface by itself is not the source ("generator") of the 1/f noise, but probably is a factor of the volume 1/f -noise "reduction".