The noise can occur when there is a heavy buildup of calcium on the heater coil. This causes the coil to get much hotter since the layer of calcium acts as an insulator and the water may come to a boil because the coil rises to a high temperature trying to heat the water.
Unless it is too clogged, this may help; empty the water out completely, then put two cups of regular vinegar in the water chamber and turn on the power for a half-minute or until liquid appears in the pot.
Let it sit overnight and then let the remaining vinegar pump through.
You will have to run several pots of water through the machine or have some really strange-tasting coffee.
There are commercial products that act more quickly and thoroughly than vinegar but also much more caustic.
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http://www.bunn.com/sites/default/files/care-and-cleaning/43460_0002d_mycafe_use_care_manual_english_0.pdf
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