  Quasi-periodic oscillations (QPOs) discovered in the decaying tails of giant
flares of magnetars are believed to be torsional oscillations of neutron stars.
These QPOs have a high potential to constrain properties of high-density
matter. In search for quasi-periodic signals, we study the light curves of the
giant flares of SGR 1806-20 and SGR 1900+14, with a non-parametric Bayesian
signal inference method called D$^3$PO. The D$^3$PO algorithm models the raw
photon counts as a continuous flux and takes the Poissonian shot noise as well
as all instrument effects into account. It reconstructs the logarithmic flux
and its power spectrum from the data. Using this fully noise-aware method, we
do not confirm previously reported frequency lines at $\nu\gtrsim17\,$Hz
because they fall into the noise-dominated regime. However, we find two new
potential candidates for oscillations at $9.2\,$Hz (SGR 1806-20) and $7.7\,$Hz
(SGR 1900+14). If these are real and the fundamental magneto-elastic
oscillations of the magnetars, current theoretical models would favour
relatively weak magnetic fields $\bar B\sim 6\times10^{13} - 3\times10^{14}\,$G
(SGR 1806-20) and a relatively low shear velocity inside the crust compared to
previous findings.
