An Etymological Dictionary of the German Language/brüllen
brüllen, vb., ‘to roar, bellow, low,’ from the equiv. MidHG. brüelen; in UpG. dialects even now briele, brüele. The remarkable short ü of ModHG. compared with MidHG. üe may be explained by the pret. brülte, where the shortness of the vowel is produced by the following double consonant; OHG. *bruowilôn is wanting; allied perhaps to E. brawl?. From the root brô (‘to scald’) in the sense of ‘to bubble’?.