Garmin GNC 355 Pilot's Manual page 54

Get Started
OPEN SQUELCH
The automatic squelch function rejects many localized noise sources. Overriding
this function may be helpful when listening to a distant station or setting the
volume level.
2-20
Tap once to override the automatic squelch function. Tap
again to return the squelch to automatic operation.
"SQ" annunciates in the COM active frequency window to
show when the squelch is overridden (i.e., when the
squelch is open).
The Open Squelch function is accessible from the
COM Volume page and slide-out menu.
Pilot's Guide 190-02488-01 Rev. B