Gundam Wing Notes #1 : Understanding Hn.
First off one must understand the philosophy of Hn. (I know I sound like an insane baka, but bite me if you're gonna flame.) Hn is not actually a grunt but a very clever acronym for "Heero noise."
Heero has manipulated the Hn industry and therefore he has many moods of Hn. I'll briefly detail a few of them here.
Hn (Hn w/out period) - Usually means either the fanfic writer or Heero is in a hurry. Intepret as Hn.
Hn. (Classic Hn with period) - This Hn has many different meanings depending on context and inflection. It can mean anything from "Get out of my way" to "You are a baka." Watch the surrounding sentence or spoken phrase carefully and if spoken listen for the tone of the speaker.
Hn? (Hn with Question Mark) - This denotes a questioning Hn. As above, watch for inflection and context to determine exactly what they are asking for or about.
Hn! / Hn!? / Hn?! (Hn with exclamation/extended exlamation) - This denotes that the speaker is surprised/mad/agitated. Listen for context, if any, to determine source of irritation and so forth.
HN. (Uppercase Hn with period) This deontes an angrier user of Hn. However, with the period it is obvious that they are making some effort to maintain control, so listen carefully for inflection and remember not to agitate the speaker.
HN? (Uppercase Hn with Question Mark) This denotes an angry and questioning speaker. Perhaps you showed up at the door while they were doing something that they wished to remain a secret...(*cough* *cough*)
HN! etc. (Uppercase Hn with exclamation) This denotes a very agitated speaker and is probably the grunted equivalent of "Omae o korosu." In other words, haul arse.