The dynamics of a trace amount of linear chains in a swollen network are investigated in the presence of excluded volume interaction and long range hydrodynamic back flow effects via Oseen tensor description. The practical interest on tracer diffusion has triggered the development of fluorescence correlation spectroscopy as a direct way to have access to the tracer diffusion coefficient. We present an approach based on slow and fast mode theories to show that the quasi elastic scattering technique provides another, indirect way to reach a wealth of information in addition to the diffusion coefficient. We invoke the blob model as a way to analyze the properties of a trace amount of polymer in a host medium made of free chains and hint to its extension to a cross-linked network. The random phase approximation in the static and dynamic versions is used to obtain the properties of the slow and fast relaxation modes, the former one being reminiscent of tracer dynamics.