The wooden stair tread shown below has become loose when moisture entered the staircase from the wall's side and warped it. The water ingress has since been fixed, but the tread often makes a loud bang noise when somebody steps on it. The staircase is made out of reinforced concrete on which the treads and risers are set. How can I fix it?
The loud bang is presumably the groove under the step dropping rapidly back onto the tongue of the riser below it.
If you can get the tread to lay flat, by making sure the tongue in the riser enters the groove, and also if you can get the tread out without disturbing the rest of the staircase, then the simplest method would be to glue it down into the correct place.
Use sandbags &/or clamps [though it doesn't look too easy to get clamps on] to hold it in place until set.
If you can't get the tread out, then take out the next riser with it, trim one or both tongues a bit before slotting it back… absolute worst case scenario, you'll have to keep stripping back the entire staircase until you reach one end, but I don't think that's likely.
I'd try @Tetsujin's solution first as that would be invisible when done, but if this still comes loose, you could consider drilling a few holes straight down into the standing board (as centerline and straight down as possible, you'll need grip on both sides) and drill a slight countersink. Then fix it in place with wood screws of sufficient length (at least twice the thickness of the step). You can cover the screwheads with some appropriately coloured wood filler when done.
As this tightens them together, the creaking should also stop. As an additional measure to both the gluing and screwing solutions, rub any surfaces where wood touches wood without being glued, with the back of a candlestick. This should remove any remaining creaks.
