I have a 1999 328i with 45k miles. The car is very noisy when rolling at low speeds. Hard to describe the sound- rumbling or maybe groning noise - especially around 20mph. Doesn't matter if the clutch is depressed or whether or not I am braking. Any Ideas?

sounds like a wheel bearing, rear bearings being a fairly problem on E36's & E46's. Find a quite street and isolate the corner of the car it's coming from in gentle left and right 'swerves'.

It is most likely a wheel bearing like jeffrey said or it can also be the tires. Have a reputable tire dealer check your tires.