Have you been getting a whiff of a musty odor in your home, particularly around a sink or drain? Have you checked, but found no obvious leaks in the cabinet or from the exposed plumbing? If so, don't rule out leaks quite yet. The problem could be a drain or water supply line that has sprung a leak inside the wall. The following guide will help you determine if this is the case.
One of the most obvious signs of an internal leak is water damage, but you have to know how to spot it. Empty out the cabinet under the sink where you suspect a leak. First, perform a visual inspection. Look for warped wood or peeling, bubbling paint if the surface is painted. Next, grab a flathead screwdriver. Gently probe the bottom of the cabinet and the wall below and behind the sink. If the surfaces feel soft, then chances are there is moisture trapped behind the wall. A plumber can cut out the water-damaged wallboard and repair the leak. Wallboard can easily be patched and repainted. Waterlogged wood and cabinetry can sometimes be dried and refinished, but replacement may be necessary if rot has begun.
A running meter
If you are still not sure if there is a leak, you can test for a water supply line leak; this won't help if the leak is in the drain line, of course. First, shut off everything in the home that uses water, including appliances like the ice maker in the fridge and evaporative coolers. Next, check your meter to see if it is still running. If so, there is a leak somewhere, which could be in the water lines running to the sink you suspect of a leak. Some leaks are slow and may not easily be detected by watching your meter, though. In this case, record the meter reading then wait a half hour or so before checking. If the reading has changed, there is a leak somewhere.
Loss of water pressure
Another sign of a leak in the supply line is an unexplained loss of water pressure that only affects one sink. Pressure or volume loss could be the result of some water leaking out into the wall. To verify, you first need to check that the aerator screen on the front of the faucet isn't clogged. A plumber can also check to see if you have a pressure valve installed on the water line, as sometimes a loss of pressure indicates that such a valve needs replacement. Failing these being the cause, a leak is a likely issue.
Contact a plumbing contractor if you need more help. They can fully inspect the system and find any leaks that may be present.