Andy Brilleaux has a very good idea regarding the speaker/headphone problem.
In direct-conversion receivers like this, there is a very large audio gain.
I'm greatly impressed that Hans has managed to keep this huge gain well-tamed.
But we users can cause problems with wiring the radio up. This involves ground
currents, and how they return to the +12V power source.
Your speaker ground line should likely return to the +12V power source ground,
rather than connect to the QCX circuit board ground.
You can wire the audio "hot" terminal from headphone jack to a speaker, but the
ground return should not be wired to the headphone jack, but return straight
back to the DC power source...awkward, but it may help reduce the disturbing
growl, or kill it entirely.