loud
He let out a loud groan of frustration.

little, slight, small, soft
deep, low
long
muffled, quiet
audible
annoyed, exasperated, frustrated
collective

emit, give, let out, utter
bite back (AmE), hold back, stifle, suppress
hear

with a groan
He stood up slowly with a groan of pain.

groan of
a groan of annoyance

moans and groans
The doctors all ignored her moans and groans.

loudly
quietly, softly
slightly
aloud, out loud
inwardly, mentally, silently
He groaned inwardly at the thought of spending another day in that place.

in, with
Some of the patients were groaning in/with pain.

at, from
He groaned at/from the pain.

moan and groan
There's no point in moaning and groaning about not having any money.