Today’s word of murky upspring is trouble.
Trouble, the Oxford Dictionary Online (ODO) has:
trouble, Middle English: from Old French truble (noun), trubler (verb), based on Latin turbidus
Online Etymology has:
trouble, early 13c., from Old French trubler (11c.), metathesis of turbler, from Vulgar Latin *turbulare, from Late Latin turbidare "to trouble, make turbid", from Latin turbidus
Is it truly rooted on Latin turbid? True enuff that the staf r often moovs about in a word (that’s the metathesis). Take thru from Old English þurh (thurh). The r and the u hav swappt spots. However, might there be a Frankish/Teutonic root?
Let's look at the German Trübal, n., ‘affliction,distress’, from MidHG. trüebesal, OHG. truobisal; an abstract of trüben. — Kluge, p369.
trübe, adj., ‘turbid, gloomy, dull, dim’, from MidHG. truebe, adj. (truobe, adv.), OHG. truobi, adj., ‘obscure, gloomy, dull’ allied to trüben, ‘to darken, tarnish, cast a gloom over’, MidHG. trueben, OHG. truoben, ‘to darken, sadden’. Comp. AS. drōf, ‘dirty, troubled’, Du. droef, ‘dull, sad’, Goth, drōbjan, ‘to confuse, lead astray, excite commotion’, AS. drēfan, ‘to disturb, agitate, trouble’. In the non-Teut. languages there are no certain cognates of the Teut. root drōb, ‘to confuse’.
Trübsal, n., ‘affliction,distress’, from MidHG. trüebesal, OHG. truobisal; an abstract of trüben. — Kluge, p369
Then there is Trübel, m., ‘confusion, trouble’, Mod HG. only, from Fr. trouble.
I don't know the root of the Norwegian and Swedish words (trøbbel and trubbel). They may also be from the French … or they may not.
The meanings of English trouble and German Trübal are more aline'd with each other than with Latin turbid. Isn't it more likely that the French trouble is from a Frankish root or other Teutonic root like OHG truobi? Thus, I don’t think there was any "metathesis" needed, the word likely alreddy stood in Frankish.