You leave the car wash and the brakes suddenly squeal, scrape, or grind for the first few stops - then go quiet. It is a surprisingly common worry, and in nearly every case it is completely harmless. Here is why a wash makes brakes noisy.
Water on the rotors
A car wash soaks the brakes. A film of water between the pad and rotor changes how they contact for the first few stops, causing a temporary squeak or chirp until the heat of braking dries them off. This clears within a stop or two - the same idea as why brakes squeak in the rain.
Flash rust forms almost instantly
Bare cast-iron rotors flash-rust within minutes of getting wet - see why rotors rust overnight. A quick coat of surface rust from the wash water makes the first few stops grind or scrape as the pads scrub it off. If the car then sits wet, the rust is a bit heavier and the noise slightly longer, but it still cleans up fast.
Normal and self-clearing
Noise that appears right after a wash and disappears within the first minute of driving is textbook harmless moisture and flash rust - no action needed. Making a few moderate stops dries and cleans the rotors and restores quiet braking.
When it is not the wash
Only worry if the squeal or grind persists long after the brakes are dry and warm, or you feel it in the pedal - that points to worn pads, not water; see why brakes grind. If noise lingers, inspect the pads and fit fresh pads and hardware if they are low.