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IDLE uses tkinter.widget.bell() in various places to signal an exceptional situation. Examples include 'Find' when the target cannot be found and 'Expand word' when there is no expansion (or are no more expansions.
[The tk manual page is http://www.tcl.tk/man/tcl8.6/TkCmd/bell.htm. I have no idea what difference it makes which window the bell is rung on.
When running test_idle, there are about 5 beeps. They serve no purpose and can only discourage buildbot owners from running gui tests, even if they could. Test_tk and test_ttk_guionly are silent.
A solution is to replace something like 'self.text.bell' with 'self.bell', where the latter is either the former or 'lambda: None', depending on a 'utest=False' parameter.
The replacement could lead to the following bonus in a follow-on issue: a separate function could be augmented to emit a 'visual bell', a signal that is seen rather than heard. This would benefit both the hard-of-hearing and those who edit with sound turned off or redirected to headphones not worn. Notepad++ somehow flashes the Search dialog title bar as well a adding a message to the status bar. I would rather flash (blink) the text not found or expanded. The idea is to replace a hard-coded external function with a custom wrapper subject to future improvement.

You could guard the noisy tests with "@support.requires_resource('audio')" (however that is actually spelled :)).

New changeset 2eb84fe85889 by Terry Jan Reedy in branch 'default':
Issue #27732: Silence test_idle with dummy bell functions.
https://hg.python.org/cpython/rev/2eb84fe85889 

Bell-using files patched (module and test)
autoexpand, parenmatch, replace, search, undo
Bell-using files not patched (no test yet) 
debugger, editor, grep, history, pyshell, scrolled_list, zoomheight
A second bonus is that the dummy bell function, instead of lambda: None, can be a mock used to test that bell() was called when it should be (or even not). This was already true for test_undo, except that the mock wrapped rather than replaced the real bell function. I opened spinoff #27733 to extend this idea.
To make this possible, I gave the class instances a self.bell function that was replaced by the test code afterwards. I am leaving this issue open to give the other bell-using classes a similar attribute in preparation for future tests.

> @support.requires_resource('audio')?
The point is that the noise is extraneous and should be optional. I want the tests to run (quietly) when I run them with unittest (which is most of the time) and anytime -ugui is true (too few of the buildbots). It was not too hard fix and has bonuses both for normal running and for tests. The parenmatch module has an option to turn off noise, and I might generalize that option, while still providing visual signals.