loud
He let out a loud groan of frustration.

little, small | deep, low | harsh, hoarse | muffled

give, let out, utter | stifle, suppress | hear

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

~ of

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

at, in
She groaned out loud in protest.

with
Some of the patients were groaning with pain.

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