Some brakes are quiet at speed but squeak, groan, or grind only when you are creeping along or almost stopped. The low-speed-only pattern is a clue to what is going on. Here is what causes brake noise that shows up mainly at low speed.
Low-speed groan or moan
A low groan or moan right as you come to a stop is often normal resonance from certain pad compounds interacting with the rotor at low speed - more common with some semi-metallic pads. If it is occasional and the brakes work fine, it is usually harmless character, not a fault - related to cold vs hot brake noise.
Surface rust at low speed
The first slow stops after the car sits scrub off overnight surface rust, which can squeak or lightly grind at low speed then clear - see why rotors rust overnight and brake noise after sitting. This is normal and disappears within a few stops.
Worn pads and the wear indicator
A high-pitched squeal that shows at low speed can be the wear-indicator tab telling you the pads are low - see the squealer tab. And a low-speed grinding you also feel in the pedal is the serious one - metal-on-metal from worn-out pads; see why brakes grind.
How to read it
Occasional low-speed groan or a rust chirp that clears = usually harmless. A persistent squeal or a grind felt in the pedal = worn pads, inspect and replace. Check pad thickness to be sure - see minimum pad thickness - and fit quality pads with fresh hardware if low.