AtD translation: a-thrum with excitement

[Mike Jing]

P255.1-6   The Inconvenience was in a remote part of the Arsenale, out of
dry-dock at last, shining and shipshape and somehow increased in size.
Chick greeted his shipmates, who were a-thrum with excitement over reports
that their Russian counterparts had been observed getting up buoyancy,
carrying on board their ship a number of mysterious crates and casks, as if
preparing for an engagement.

What does "a-thrum" mean?