Water heaters. Gurgling and clunking may sound bad, but they are symptoms of mineral deposits in the bottom of the tank, not of imminent failure. Deposits will reduce the heater's energy efficiency, but in most cases can be removed by flushing the system. Drips are another matter—and may give you a few days or even weeks of warning when an aging, rusting hot water tank is about ready to let go. There are likely to be telltale rust-colored drips around the base of the tank for a start. But when the drips become constant, you're on borrowed time, and the system should probably be shut down until a replacement is installed.
Paint. Cracking and peeling exposes siding to the weather, and can foster leaks and rot that lead to major repairs. Early warning of cracking and peeling often takes the form of a surface pattern called alligatoring. At first, you'll see small hairlines forming at right angles, then spreading out to form box-like shapes. That's the red flag, and the time to prep and repaint.
Asphalt shingles. Shingles are protected with a top layer of granules that gradually wear or wash away over the years. Losing some won't hurt. But eventually, granule loss exposes the underlying asphalt to sunlight and weather that make shingles brittle. Look for the red flags, like piles of granules on the ground where gutters empty, and dark patches of asphalt unprotected by granules on the roof. As the dark patches expand, exposed sections of shingles are more likely to break off, exposing nail heads underneath and fostering leaks. Before that happens, reroofing is usually possible without stripping the original roof.
Shifting foundations. Keep track of large or deep cracks on foundations, and bulges or depressions in any masonry wall. Once you spot a deformation like staircase-pattern cracks in a concrete block wall, document the problem. For a contractor, to understand what's happening and recommend the right fix, it's vital to know if the crack is active, and if so, how fast it is moving.