I think it depends on your threshold of "substantial." A human brain responds in a complex an (probably) noisy fashion to inputs from the rest of the world. That I might choose to flip coins and choose actions based on the outcome is part of the operation of my future thoughts and actions. In my case, I would choose random numbers based on complex and noisy physical operations. For example, the 4th decimal place of a voltmeter reading the voltage across a hot resistor, and to make it fun, I would take the 4th place at exactly 15 seconds after the beginning of the most recent minute, suppose it is N, then take the 4th place N readings later, call it M, then take the 4th place M seconds later, call it Z, this would be my random number. I would use the 4th place only if I saw it was one or two to the right of where I saw variation on the voltmeter. ALL of this, the operation of my mind in deciding to do this, and the physical details of the voltmeter-hot resistor system in detail so as to predict the resistor's detailed brownian motions, AND it's interaction with the voltmeter. you'd probably have to predict how I would pick the resistor and the voltmeter to predict what would happen, and as I considered what I would do I would pick the 17th voltmeter on a google search page. I would reach into a bin of resistors and pick one from the middle. I would partially smash the resistor with a hammer to make further difficulty for anyone predicting what would happen.

SO all of that has to be predicted to come up with Z, the output of my random number generator based on a resistor and voltmeter, Is that "substantially" harder than predicting a single coin toss, or is it somehow "substantially" similar?