Automatic gain control (AGC), is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the input.

Every radio schematic I have looked at that had an RF gain control used the RF gain control to override the AGC voltage. The reason you hear so much quieting is that reducing the effectiveness of the AGC lets the noise floor sound as weak as it really is

* How this AGC works: on any time, R6 voltage drop almost always 0.7V, so Q1 base voltage drop to 0.6V when AGC voltage(on C7) drop to zero. on extreme circumstance, i.e Q2 oscillation, AGC voltage became -0.6V, Q1 base drop to 0.2V, and Q1 cut off.