Congratulations on your 'new' car. Welcome to the forums.

Brake rumble will occur when a vehicle has been sitting for a long period of time. As the Sales guy said... dirt and debris as well as some surface rust (may not even be very visible). This will get worn off with the brake pads. Not a bad idea to have the entire vehicle 100+ point inspected when you get the oil changed at the dealer service center.

The rest is below...