The polysiloxanes are polymers of choice to study the various dynamic processes by dielectric relaxation, photon correlation spectroscopy, and other dynamic light scattering techniques in bulk and in nanoconfinement and at elevated pressures. These studies of poly(methylphenylsiloxane) and poly(methyl-para-tolyl-siloxane) (PMpTS) have found the presence of the so called alpha' relaxation slower than the segmental alpha relaxation. Despite the advances made, there is a dearth of mechanical measurements to reveal the viscoelastic properties of the alpha' relaxation. Dynamic shear modulus measurements had failed to resolve the alpha' relaxation. We made shear creep compliance measurements on a high molecular weight PMpTS and were able to resolve the alpha' relaxation. The data enable determination of the compliance level contributed by the alpha' relaxation and the temperature dependence of its characteristic relaxation times tau(alpha)'(T), which is weaker than tau(alpha)(T) of the alpha relaxation. These viscoelastic properties of the alpha' relaxation enable us to demonstrate that it originates from the sub-Rouse modes found also in other high molecular weight polymers polyisobutylene, poly(methyl methacrylate), and polystyrene. Our creep compliance data were compared with photon correlation spectroscopy (PCS) data obtained for the same PMpTS sample. Despite some discrepancy in the relaxation times from the two techniques, we verify that the alpha' relaxation observed by PCS also originates from the sub-Rouse modes.