The role of viscous stress fluctuations in a flow in the process of th
e generation of aerodynamical (hydrodynamical) noise is considered. Ne
w forms of Lighthill's equation and Curle's integral are derived. On t
his basis the conclusion is made that sources of sound due to viscosit
y are proportional to the divergence of a velocity field and this fact
causes the viscous sources to be ineffective at low Mach number. Furt
hermore, it is shown by simple example that the ''viscous sources of s
ound'' in Curle's integral are false, and they arise because the visco
us terms are mistakenly placed on the right-hand side of the initial w
ave equation.