Humming Noise

Haved owned my home for 3 winters now and have noticed that in the winter, the pipes humm when the water is run--stops when water is turned off. It seems to be louder when some faucets are on, but it occurs when any faucet/toilet/dishwasher/washer machine etc. is running. Humming is very soft to non-existant in the summer. This past winter I made the stupid mistake of leaving the garden hose attached to the outside faucet. And, the humming this past winter (and even now that the weather is getting warmer) is louder than it has been the previous 2 winters. Any idea what might be causing the humming? Is there an easy remedy? Thanks for the help!