HVAC System malfunctioning on 2003 Toyota Matrix

A-C not blowing icy cold air but rather fluctuating from "cool" to "warm" depending upon rate of speed. Warm air only when idling, cooler air when cruising between 55-80 MPH, but steadily-fluctuating between cool-warm. Engine emits loud noise when started for first time/daily, but usually ONLY emits it once upon initial start-up, and during hot weather--above 80 degrees-F.
Technician recently determined HVAC system had too much freon, thus evacuated excess, thereafter--loud noise began emitting from engine compartment upon initial ignition during hottest part of day (West Texas).

Asked by for the 2003 Toyota Matrix