  In a full QCD lattice study with $N_f = 2$ Wilson fermions, we seek to
optimize the signals for the disconnected contributions to the matrix element
of flavour-singlet operators between nucleon states, which are indicative for
sea quark effects. We demonstrate, in form of a fluctuation analysis to the
noisy estimator technique, that -- in order to achieve a tolerable signal to
noise-ratio in full QCD -- it is advantageous to work with a $Z_2$-noise source
rather than to rely only on gauge invariance to cancel non-gauge-invariant
background. In the case of the $\pi$N $\sigma$-term, we find that 10
$Z_2$-noise sources suffice on our sample ( about 150 independent QCD
configurations at $\beta = 5.6$ on $16^3\times32$ with $\kappa_{sea} = 0.157$,
equivalent to $M_{\pi}/M_{\rho} = 0.76(1)$), to achieve decent signals and
adequate fluctuations, rather than 300 such sources as recently used in
quenched simulations.
