It is perhaps worth noting that S. *_brui_ 'loud' (as in _Bruinen_ 'Loudwater')
bears a striking resemblance to the Esperanto verb _brui_ 'make a noise' (in
which _-i_ is the infinitive ending; the adj. form is _brua_ 'noisy, boisterous,
Tolkien, of course, was well acquainted with Esperanto, writing to _The British
Esperantist_ in 1932 that "I know [Esperanto], as a philologist would say, in that
25 years ago I learned and have not forgotten its grammar and structure, and at
one time read a fair amount written in it"
The Esperanto verb was taken from Fr. _bruire_ 'to make a noise, roar', whence
also _bruit_ 'noise, din, clamour, sound', a word that has also been borrowed
by English. According to the OED, _bruit_ is thought to derive from L. _rugîre_
'to roar', and "the prefixed _b_ may be due to some onomatopoeic alteration".
An alternative theory is that the prefixed _b_ is due to the influence of LL.
_bragire_ 'to cry out'. Whatever the case may be, Esp. _brui_ and Fr. _bruire, bruit_
were perhaps influential in the creation of S. *_brui_.
The association of _bru-_ with 'noise' in Tolkien's mind seems also to occur in
the Gnomish Lexicon, in which we find Gn. _brum_ 'noise' and _brumla-_
'make a noise', cited as variant forms under the entries for _rum, rumla-_. The
addition of _b_ in the variant forms may be an onomatopoeic addition (as the
OED suggests with _bruit_), perhaps suggested by German _brummen_ 'mumble,
grumble, growl' (whence the rare English verb _brum_ 'to murmur, hum'). The
unprefixed forms _rum, rumla-_ are perhaps the conceptual antecedents of
Q. _rúma-_ 'shift, move, heave (of large and heavy things)' seen in the late
version of "The Last Ark" (MC:223).
-- Patrick H. Wynne
[Note too the play-words "brum" and "vroom" used onomapoetically of the rumbling
sound of engines. CFH]