Older plumbing might have a metallic aftertaste due to copper and iron corrosion.  
Hot water, poor plumbing, or stagnant water conditions (as when a house is closed for lengthy periods of time) can accelerate plumbing corrosion. Metallic tastes can also emerge when new plumbing or fixtures and metallic materials have not settled. 
 
Prince William Water's suppliers, Fairfax Water and City of Manassas, add a corrosion inhibitor to prevent metals from seeping into drinking water. Run cold water for a few minutes to freshen your home plumbing if your water tastes metallic.