Squeaks and rattles are very hard to diagnose. In your case the rumble happens at very low speed, this makes it easier. I would advise to have someone standing near the lip on your driveway when you drive over it and try to find out where it comes form. The driver should concentrate on driving so the listener does not get run over.

If that does not work you can try to find a shop that does shock testing with vibrating plates under the wheels.

Possible causes for this rumbling are:
-Hood alignment / vertical adusment screws
-Lose front fender
-Fan hitting the radiator shield
-Battery not properly secured
-Air filter housing
etc
Rob