Yes, I believe that it's a combination of both High Pass Filter and Low Pass Filter in Audacity. Just be careful, as the volume will decrease and will need to be raised. ;)

I'd be interested in knowing the process too. What values did you use if you did use audacity author?

This is really late, but I have the values necessary to adding an HECU Radio Filter effect to your voice.
The audacity settings that were used are:
Equalization - 24dB
High Pass Filter - 1000 Hz, 12dB rolloff
Change Pitch - -9.979

This is really late too! Thanks ;P

Did you by any chance use a leveller effect? Sounds like it.

No problem. Glad I could help.
The method I used is a 1:1 method of recreating the effect that Starscre4m used in his mod. I didn't use any leveller effect, though that could be another way of achieving the sound.

Oh strange, I followed your steps but there was a noticable difference in the sound. But i applied a heavy leveller effect and the distorted feel came too it. :p