Check your setting for the SQL/RF, it can be set for either;
This is on a 817 but i would think the 818 would be the same apart from the menu number.
SQL/RF Knob
In the USA version, this (outer) knob adjusts the gain of the receiver’s RF and
IF stages. Using Menu Selection 45, this control may be changed to function as a Squelch
control, which may be used to silence background noise when no signal is present. In
the other versions, its default setting is set to “Squelch.”