Yes, you get the noise from the U-Net itself.

The idea behind DDIM is to make the reverse process deterministic, i.e. conveting the SDE into an ODE (eq. 14). Now that said, an ODE can be integrated backwards in time starting from final solution (the clean image x_0), integrating with negative `dt`, reaching at noise (i.e. "encoded feature") x_T. Thus, you get a negative sign in front of the nosie-estimator `\epsilon_{\theta}` and then treat it like a normal ODE and integrate from end-time (t=0) to start-time (t=T).