Here is an oddly specific but very common complaint: a squeal, scrape, or rattle from the brakes that is there while driving but STOPS the moment you press the brake pedal. That on-off pattern is actually a useful diagnostic clue. Here is what it usually means.
The classic case: worn-pad wear indicator
The most common cause is the wear-indicator tab. When the pads get low, the little metal tab drags on the rotor and squeals - but when you press the brake, the pad clamps tight and holds the tab still, so the squeal stops; see the squealer tab. A steady squeal at speed that quiets when you brake is a textbook worn-pad signal - see when to replace pads.
Loose pad or hardware rattle
A rattle or buzz over bumps that stops when you brake points to a loose pad with too much clearance - see brake pad gap noise. Off the brake, the pad has room to vibrate; pressing the pedal clamps it firmly against the rotor so it cannot move, silencing the rattle. Fresh anti-rattle hardware fixes it.
A light dragging noise
Sometimes a faint scrape from a slightly bent dust shield or lightly-touching pad changes or quiets under braking as the assembly flexes - see brake dust shield rattle. Usually harmless, but worth confirming.
What to do
Because the top cause is worn pads, a noise that stops when you brake is your cue to check pad thickness soon - see minimum pad thickness. Replace worn pads and refresh hardware with quality pads before it turns into grinding.