[TenTec] 555 NB - is it working?

[Brian]

How can I tell if the noise blanker in my 555 is working? I don't notice
much of a difference in the audio when I engage it, except that I hear
this faint whine in the background. Like a whistle, just not overly
loud.  I flip it to the OFF position and the whistle goes away.

Bought this 2 weeks ago from Ten Tec used but I'm not sure this NB
thingie is working. It's not appearing to be blanking any noise.

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  KB9BVN :NORCAL #2792 FISTS #5695/CC# 764 QRP-L #1540
        39.558 N   86.095 W   Johnson Co., Indiana
     GRID: EM69WN - TenTec Scout - Attic Dipole - 5w
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