Lamps are NOTORIOUS for sucking the humidity out of the cage. Screen tops also allow too much airflow, also decreasing humidity.
By not using the digital thermometer/hygrometer, I'm assuming you're using the dial one? I hate to say it, but those are also notorious for showing incorrect humidity..
I think one of the most important points to bring up is the use of that heat pad. How are you regulating it? Is it just plugged in, or do you have a thermostat?
All these questions regarding temp and humidity are to help try and rule out a possible cause for an RI (respiratory infection). Improper temps and humidity can lower the python's immune system which can directly cause an RI. Getting the humidity and temperature to an ideal level will help boost the immune system.
A "wheezing" sound can also be caused by some loose shed around the nose. If the humidity was at a good level during a shed cycle, you wouldn't have to worry about this.