A hollow trunk is not automatically a removal order. Trees can remain standing with internal cavities, but the location and extent of decay, remaining sound wood, cracks, loading and targets must be considered together.
Why trees become hollow
Decay can break down internal wood after injury or other entry points while living tissues continue to grow around the outside.
What makes it more concerning?
Large openings, expanding cracks, basal decay, fungal fruiting bodies, root problems, significant lean or a cavity associated with a major union can increase concern.
Can you tell by knocking?
A sound difference may suggest a cavity but does not quantify structural strength. Professional assessment may use visual examination and additional diagnostics where justified.