If a monster steps on a squeaky board near you when you are asleep, it may wake you up. (Really, this should be the case for any effect capable of waking monsters.)

Monsters, including the player, under a certain body weight plus inventory weight don’t trigger squeaky boards. This weight factor is fairly light, possibly in the low hundreds (so even a player carrying nothing will still trigger it).

Stepping on a squeaky board near a sink may spawn one or more sewer rats on the sink. Alternatively, sinks may occasionally just spawn a sewer rat on their own.