Landing sound playing both my custom sound and the default "GRUNT" sound effect?

You could add a sound file called DSOOF to your pwad, and have the sound file completely silent.

This of course will also override the nudge sound you would hear when the player searches to open a door etc.

If you still hear the sound, then there's another version of the sound floating around, perhaps in the engine resource file.

DSNOWAY is the sound for pushing on a wall, and DSOOF is the sound for landing.  In the Doom IWADS, they are identical, but you can replace them independently, with no need for SNDINFO.

I can't take credit for this answer. I totally forgot about he DSNOWAY sound!

Still the same method I suppose.