My ABS system is engaging why?

Our vehicle is making a noise that I can only describe as what it sounds like when a motorcycle revs up. Sometimes it is very loud (turns heads, in fact), and other times, not as loud. First, it only happened on right turns, but not ALL right turns. My foot is not on brake, but I am initiating the turn at only about 10 mph. It now occasionally happens on left turns as well. Yesterday I went directly over a speed bump (again, no brake, about 10 mph) and it happened. It seems like possibly some kind of vibration coming from the front end or underneath the car. We have had both front wheels off, checked for loose brake shoes, etc. Checked under hood and under car for anything else that may be loose. Also, the car is not "stuck" in four wheel drive. Someone suggested that it may be an anti lock brake sensor going out? It's a little bit of a stretch, but I suppose the noise could be similar to what it sounds like when you brake on ice before the anti locks kick in. Please confirm or any other ideas would be much appreciated. Thank you.