How can I identify the start and end of lower period of noisy data?

I have noisy data at roughly 1 minute intervals across a day.

Here is a simple version:

How can I identify the start and end index values of the less noisy and lower valued period marked in yellow?

Here is the test data:

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

arr = np.array([8,9,7,3,6,3,2,1,2,3,1,2,3,2,2,3,2,2,5,7,8,9,15,20,21])

plt.plot(arr)
plt.show()

from How can I identify the start and end of lower period of noisy data?