Mmmmmm ...
A bit more information would be helpful ....
Also a "search" would have no doubt revealed that this problem has been attacked on this forum before.
My guess here is that your microphone impedance is incorrect for the avioncs install on the "fault" aircraft .....
What type of mic do you have installed on you headset .... if you have the "dynamic mic" (without the amplifier) install you might try an "amplified dynamic mic" in its place ... this may cure the "fault".
If your headset is a standard commercial use set the actual h'phones will no doubt be 700 ohm impedance units ... if set up for military a/c they will be 8 ohm units ... but ... I doubt the wireing of your headset is the problem ...unless your "avionics specialist" had been at work on it.
I'm a bit worried that your "avionic specialist" could not diagnose this fault for you .... you might want to consider a change there!
Cheers
BTW ... wire diagrams for Dave Clark headsets are available off the Dave Clark website ....look em up!