At slow speeds, I hear a low motorized humming sound coming from the engine. It last for about 4 seconds then goes away. It comes back on when it feels like it. Again for about 4 seconds.

Anybody know what that is?

 

I'll vote the ABS pump. Does it do it when you start the car?

 

second that,, my ABS pump make a short humming noise, you probably will hear it immediately every time you crank your car.

 

Is it something I have to worry about?

 

i third it. on ur older car especially mite do it more often if the accumlator isnt holding pressure. if it gets to a point where its too bad, ur ABS wont work.