Your Endpin appears to be creaking by way of sympathetic resonance or vibration. As you noted certain notes cause the creaking while others do not which indicates that there is resonant frequency that the Endpin is reacting too.
The best advice would be to have you take the instrument to a respected luthier for inspection and possible repair. I say 'possible' as this might be the case of a badly designed instrument which may make a repair impossible.
In the meantime, a simple workaround other than removing or cutting off the Endpin, would be the opposite. That is add more mass to the pin to change its resonant frequency. You could do this a number of ways and still make this modification reversible so you do not make any permanent changes to the instrument.
One way to do this would be to use a C clamp but add felt to the edges that engage the Endpin, or a Spring Clamp (be sure that the teeth are coated with soft plastic), or a Handscrew Clamp.
If your Endpin is removable then by all means try something lighter or heavier with the same diameter to see which item is less likely to creak.