Understanding the normal phase behaviour of the iron based superconductors is very important in order to shed new light on the physics of these complex materials. Mn impurities substitution on the Fe site in optimally electron doped La1-yYyLaFeAsO0.89F0.11 leads to a very rapid drop of Tc so that even a small amount of Mn is enough to completely suppress the superconductivity. 19F nuclear magnetic resonance spin-lattice relaxation time (1/T1) revealed the presence of very slow (MHz range) spin fluctuations, at temperatures above Tc, whose amplitude increase with Mn content. We thereby propose a ZF and LF muon spin rotation investigation of the low energy dynamics developing in the normal state of these materials to complement our 19F NMR studies.