a drilled tank, set up correctly, can be completely quiet.
btw, just return the cpr overflow box.
the only thing i would keep in mind is the return line back siphoning (this can happen with both a HOB
overflow, and a drilled tank). when the return pump shuts off, the return line being filled will reverse, and start siphoning back into the sump. you'll need to do something to break siphon as soon as possible when this happens.
what i do is drill a hole in the return just below the water line. when the power goes out, it only drains a tiny bit and then sucks air through this hole.