How to find the background noise level of my impulse response measurement?

Set the time window to the last part of the impulse response, using for example the last 0.2 seconds of the impulse response which should give a good result. We assume that the impulse response has died out and we have only stationary noise in this last part of the impulse response. Now set the scope mode to Stationary signal as shown below since the noise is stationary.