Initialize Hi Pass Filter
See related question: Is initializing a digital filter&#39;s output with no &quot;momentum&quot; a non-trivial task?

The easy way is to subtract, prior to filtering, from each input sample the first input sample. This will eliminate the step in the beginning, avoiding the wavy step response. That's roughly what the other original poster said they did. Or because you have noise, you can take the average of the first few input samples as a better estimate of the DC bias and subtract that from each input sample, prior to filtering.

If you want to save the work of subtracting then changing the filter's state variables in the beginning is the way to go, and my answer to the linked question shows how to do that (with $x$ as the estimated DC bias and $y=0$ as you have a high-pass filter with zero DC response but this can be validated). Not sure if you can easily touch the state variables in your prototyping environment without rewriting the filter code.