Since the oplev beam reflected by the input mirror was very high as reported in elog2798, I tweaked a steering mirror just after the oplev laser source for input mirror (Fig. 1). The oplev sum for input gets around 5000.

Fig. 2 shows the oplev spectrum of input mirror (top right). There are large peaks at 2 Hz and 3.4 Hz. We need to open the input chamber and check the suspension.