Why do you never put your foot on the accelerator pedal while shifting?

When you shift gears the gears must be allowed to slow down to sink.
To shift do the following;

Remove foot completely from accelerator pedal.
DO NOT USE PARKING BRAKE.
When RPMs drop depress the clutch pedal fully.
Shift into next gear gently.
Let up off the clutch pedal and this may take some learning to release it smoothly.
Never press the clutch while in gear and in motion.

You may be hearing the gear Synchros damaged by the high revving while shifting gears or high load from using parking brake.