Low rumble does it get louder when you drive faster?
If it does it might be ball joint or wheel bearings?
Top mounts are more noticable going over pot holes and rough roads where the spring keeps hitting the top.
Problems with top mounts its only noticable if weights on them, but the ball joints and wheel bearings you can check with just spinning the wheel and listening.
Ball joints wiggle and see if there is play in them (they will move small amounts but not enough to feel like a space or metal hitting metal)