Water sputtering and hissing from all upstairs water outlets

I have a 2 story home with an upstairs bathroom. Every water outlet (sink, bathtub, toilet) in this bathroom sputters and hisses when the water is turned on. After a few seconds (20 - 30) it gets better but continues to do it occasionally during use. The downstairs bathroom does sputter occasionally, maybe once a month or so) but not often. I haven't heard any "hammering" in the pipes. It would appear to be air in the pipes but how does it get there? and how do I get rid of it?
Thanks for any help it will be greatly appreciated.
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emcconne wrote:

If the service pressure drops momentarily, air will be sucked in at the highest level. Then sputter out on the next use.
If this is a well pump, useage may draw pressure down enough to cause it.
If city water, could be pressure drops at night or during maintenance, or insufficient mains pressure/flow.
Jim
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My guess is that it is air trapped in the water. If you are on a well, you may have a slight leak before the pump. Also it is possible to just have air in the water naturally. It could also be methane gas if you are on a well. When the water sits in the pipes unused, the air seeks the highest point of your system, the upstairs. Check with a plumber, if all else fails you maybe able to get a air bleed installed simular to what is used on boiler heating systems. Greg
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