David B Williams wrote:
" In Iceland, they share the Nordic habit of taking a quick sip of breath to anticipate or signify "yes". I heard on the radio a few days ago that this is quite common in Sweden and similar places. I had to consciously resist not adopting this mannerism.
I can confirm that mannerism. I've been driving around a lot in all Nordic countries. It's common in Norway, Sweden, and Finland. Interestingly, I have not noticed it in those communities in the far North of Sweden where Same live. 

A particular case is Estonia. My business partners there are of two kinds : the "real Estonians", those who have Estonian as their native language and have lived there for many generations, and those who have Russian as their first language. Interestingly, the former have the mannerism, the latter haven't, although some of them have been living in Estonia for three generations and speak Estonian fluently.

As far as I remember, Latvia does not have that Nordic habit, although they consider themselves - at least the Latvian speaking majority - as part of Scandinavia. But Latvian, just as Lithuanian, belongs to the Slav group of languages, not the Germanic group or the Finno-Ugrian group (to which belong Finnish, Estonian, and Hungarian). Neither does Lithuania, but that country is traditionally already closer to Poland than to the Nordic states. Lithuanian is a Slav language as well. 

The above suggests that there is either a cultural or a linguistic cause of the habit, or perhaps a mix of both.

As to its cause, I can only guess. My best guess is that real Scandinavians, be it of the Germanic or of the Finnish/Ugrian type, are a rather taciturn lot, just as the Khor :

the Khors are a sensitive people. Do not speak to them; pay them no heed except from necessity, in which case you must use the fewest possible words. They consider garrulity a crime against nature.

Most Norwegians, Swedes, Fins and Estonians are of a similar disposition. Drawing in your breath to signify assent makes words superfluous. In business negotiations, I often anticipate and make use of this, for instance in the following way : "Now this is an interesting article. I think it's well suited for the Estonian market. So are the colors. And at EUR 4.75/m2, it's set at quite a competitive price." Then, more often than not, the business partner would take a quick sip of breath, signifying agreement. Nothing more need be said. Business concluded!

Twenty years ago, I would have waited for some more explicite verbal agreement, but now I know, that none will be forthcoming, and that the quick sip is all that is needed.

I picked up the mannerism myself when living in Sweden for some time, although it took me about half a year. I'm quite surprised that you had to resist it after only a couple of days. It's not so easy to acquire, but doing it the right way also signals to your partner that you know and respect his cultural habits. It often happens that during one negotiation, I draw in my own breath at least five times. Saves a lot of talking.

Ambulo, ergo sum!