When I change my instrument sound in HALion from the default to general MIDI sounds, I get a very low pedal tone as a result of the notes that HALion uses for expression maps. How do I get rid of this?
If you haven’t already, try applying the Default setting to that channel in the Expression Maps.
Those low notes are keyswitches (pitches outside the normal instrument range designed to trigger effects like pizz or arco). You need to find an Expression Map setting that filters them out.
Try switching to the the ‘HSSE (Elements)’ playback template, as that uses sounds that mostly don’t use keyswitches.
To expand a little on what Paul says, if you choose the ‘HSSE (Elements)’ playback template, what will happen is that Dorico will reload all of the sounds in your project to use the default General MIDI set within HALion Sonic SE, which will undo any changes to the sound assignment you have already made. But the advantage of doing so is that in general none of the sounds used by the ‘HSSE (Elements)’ template have keyswitches that require special expression maps, so when you then change the sound in HSSE, you don’t have to worry about going to the Endpoint Setup dialog in order to change the expression map for the changed sounds to ‘Default’.
For more on changing sounds manually, see this video.