You guys might be right... It does seem like a solenoid pulsing-feeling. The only thing that keeps me from being comletely on board is that the ABS light doesn't come on. I think the sensors and lights are working right since when I turn the ignition on, the light goes on and off to show that the light is working and then I do hear the abs solenoids do their thing the first time I go past 3 mph or so. The only thing is that the mysterious sound doesn't sound like buzzing solenoids, like when ABS is active. It's a more rythmic thud, 3 seconds, thud, etc.
I'll ask my mechanic to read the codes next time i'm in. I just worry that since it isn't very reproducible that I might not get a good diagnosis if the codes don't show up with anything.