This is a unity negative feedback system. For this system to oscillate at some frequency, we need two closed loop poles on the imaginary axis. The characteristics equation (C.E) of the system is shown below:

We can use Routh-Hurwitz table to find out these poles.

By solving the auxilliary equation (A.E) we will get the location of the poles and the frequency of the oscillation.

Hence, it can be said that the system oscillates at a frequency of 1 rad/sec.