Hi Rory, the fact that your starter is rising means that it's bubbling. The rise is caused by fermentation bubbles that are caught and held in place for a time by the gluten structure that has formed within the starter. Typically a starter will rise to a high point and then slowly start falling. The starter falls because fermentation is slowing down, and the structure of the starter is also beginning to breakdown. The bubbles that have caused the starter to rise gradually deflate and the gases rise to the surface. If left long enough, the starter will eventualy return to the level it was right after you fed it, although you may notice some frothy bubbles on the surface. 

Because you've been feeding your starter with a combination of bread flour and whole wheat flour, rather than unbleached all-purpose flour, your starter is probably thicker in texture, which will affect both the way it rises and falls, and how the bubbles appear. This is because both these flours absorb more liquid than unbleached all-purpose flour does. A thicker starter may rise a bit more slowly and may not double in quite the way we describe, but there are definitely still bubbles being produced. While there is nothing wrong with the way you're maintaining your starter, you might want to consider feeding it unbleached all-purpose flour instead of bread flour. The reason I suggest this is that AP flour has a bit more starch in it than bread flour does, and since starch is what the starter is consuming and fermenting, it will be getting a slightly better meal with AP flour. The consistency of your starter will also thin out a bit when you transition to a combination of AP flour and whole wheat flour, and you may be able to observe the starter rising and beginning to fall in the way that more closely resembles the bench marks we describe in our starter recipe. 

Otherwise it sounds like you're starter is doing fine, and feeding it twice a day at your room temperature is working well for you. I would continue with this routine for a few more days, and if you don't want to switch to AP flour, consider including a bit more water in each feeding, so that the consistency of the starter is a thick, but easily stirrable paste right after you feed it. When the starter is fully risen and ripe it should have thinned out to the consistency of a thick pancake batter. It really sounds like your starter is close to being ready to start baking bread with, but I would look for it to double in volume about 6-8 hours after each feeding as a sign that your starter is fully developed. If you don't already have your starter stored in a clear, quart-sized wide mouth jar (like a Mason canning jar) this type of jar can make it easier to see this doubling behavior. 

I hope this helps! Let us know how it goes! 

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