Given the time of year and my home's shoddy heating, I opted to put my starter on top of the clothes dryer to keep it warm. With five people in my family it gets regular use, especially towards the end of the week, so the actual level of warmth isn't an issue. But I am a little leery of the agitation- will the dryer's vibration affect the starter in any way?
The vibrations will have an effect:
On you, because it will be harder to judge the "ripeness" of your refreshed sourdough. You often want to catch the point "just before its starts to go down again" - which will be hard to do when the shaking machine bursts the bubbles all the time.
I doubt that the yeasts and bacteria in the starter will mind - they won't get seasick.
I don't think it will bother your starter any, and as far as YOU go, I read a great tip of putting my starter in a canning jar, or a glass jar, so the action of it could be easily visualized. Right after stirring down, adding air and feeding it, put a rubber band on the jar at the current level, and have one or two more rubber bands handy...I keep mine up by the top of the jar. I move one to around where a double quantity would be... example: If I have, for instance, 2" of starter in the container, My next rubber band will be at about 4", or double. I also put one at around 6", and I have an absolute visual on what that starter is doing, just by glancing in it's direction. Going up? When I first notice that it has marks on the side of the jar where is has begun to shrink, I move a rubber band to that point, and/or do my 'float' test if I am wanting to bake. Or, I know to feed it again before it drops all the way back down to the starting point.