Noisy Radiators

Hi guys, The upstairs radiators in my house have been making noise for some time time. I was windering what could be causing it. It sounds like "wush-wush-wush-wush" mixed with "tap-tap-tap-tap". I have tried bleeding them ut to no avail. The downstairs radiators are fine.
1) The noise is as described above. 2) It only affects the upstairs 3) The nois level changes 4) It only gets noisy if the central heating is turned off. It's silent when the heating is on.
Any suggestions would be welcome. Thanks Alan.
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My central heating system is hot water.
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You have air in the radiators.
1. Check the pressure in the boiler. It should be about 12 pounds for a one or two story house.
2. Turn off the circulating pump.
3. Bleed the air out of the upstairs radiators until just water comes out.
4. Turn the pump back on.
If this is a recurring problem have a plumber or steamfitter check out your air control/air elimination system to see if it is designed and working correctly.
Mark
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