Basement floor repair

Almost always because the concrete slab underneath is not flat there. A floating floor spans a low area rather than following it, so stepping on that spot deflects the planks into the void and produces a hollow click. The flooring is behaving exactly as designed; the fix is levelling the slab in that area, not doing anything to the planks.

It can be, and it is worth checking properly rather than treating it as an air-freshener problem. Moisture moving up through a concrete slab can be held against the underside of the flooring where it cannot evaporate, and a musty odour is frequently the first sign of that, well before anything is visible on the surface. Finding out where the moisture is coming from matters more than the flooring.

It is the white, powdery mineral deposit you sometimes see on concrete, left behind when moisture moves through the slab and evaporates at the surface, carrying dissolved salts with it. The deposit itself is harmless. What it tells you is not: it is direct evidence that moisture is moving up through your slab, which is worth understanding before putting flooring over it.

Solid hardwood directly on a slab is generally a poor idea because of the moisture exchange, and most manufacturers say so. Engineered hardwood, luxury vinyl plank and laminate over a proper vapour barrier are much better suited to below-grade conditions. If you already have solid hardwood down there and it is cupping, the moisture position is almost certainly the reason.

Often substantially. Older Ontario basements frequently have slabs poured without any vapour barrier beneath them, because that was not standard practice at the time, and some have no slab issues at all while others pass moisture continuously. We would rather establish what you actually have than assume based on the age of the house.