Francois2010 wrote:Thank you!
Note this: it does not occur with string instruments. What do you say to that?
Your hearing is quite excellent! 
There are explanations for what you hear, so the key is to discover how to identify them, for sure . . .
For example, after listening to the audio produced by the Notion 3 sample score you provided, I immediately recognized one source of the problem, and it is is very easy to correct . . .
Switch to the Notion 3 Mixer for the piece, where you will notice that you have the Notion 3 Reverb on the Master output track set to its default values {Dry/Wet = 50, Room = 50, Damping = 0, Predelay = 25} . . .
If you remove or disable the Notion 3 Reverb or set all its parameters to zero, then the spurious note is
not heard so loudly, which as explained below tends to suggest that it is a characteristic either (a) of the room or studio in which the instruments were played and sampled or (b) of the instrument itself, where the behaviors of the Notion 3 Reverb tend to have an amplifying affect on the spurious note . . .
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NOTE: I really like the Notion 3 Reverb, but I tend not to use it on the Master stereo track, and for the most part I do not use it at all, since I prefer to record the Notion 3 computer-generated audio in Digital Performer 7 in as dry a way as possible. I would be very happy if the Notion Music folks packaged Notion 3 Reverb as a standalone VST plug-in, and I certainly would purchase it as a standalone VST plug-in, since it is a very melodic reverberations that is both nice and easy to use . . . ]
Specifically, (a) stringed instruments and open air column instruments behave differently from (b) closed air column instruments, where the former produce all harmonics while the latter produce only odd harmonics, but brass instruments while being closed air column instruments are enhanced and modified to be able to produce a fuller range of harmonics, so it is not unexpected that string instruments will behave a bit differently from brass instruments and closed air column instruments like most clarinets, oboes, bassoons, and so forth . . .
Additionally, the ways that sampled sounds of instruments interact will depend on a virtual festival of things, but all of them are governed by the fundamental rules of physics, and from another perspective it
is not the least bit unusual for the particular acoustic behaviors and characteristics of the rooms in which instruments are played and recorded to have additional affects with respect to sampled sound interactions, where I certainly can envision the scenario where a bit of room resonance which for a single instrument sample might be negligible could become quite noticeable when several instruments all played and recorded in the same room are combined in a score, since one of the biggest problems in the digital universe is the additive phenomenon of hiss, background noise, and so forth and so on, which includes the acoustic resonance behaviors and characteristics of the rooms and studios where the instruments were played and recorded . . .
Specifically, a room or studio can have what one might call "hot spots", where the room itself tends to amplify certain very specific pitches, and similarly a room or studio can have "cold spots", where the room itself tends to diminish certain very specific pitches, so as noted it is entirely possible that some of what you are hearing derives not so much from the particular instruments as from the acoustic behaviors and characteristics of the rooms and studios where the various instruments were played and recorded . . .
The acoustical architecture aspects ofmusic rooms, concert halls, recording studios, and listening rooms are not simple matters, at all . . .
One of the more fascinating architectural acoustic phenomena is the "whispering gallery", really . . .
http://en.wikipedia.org/wiki/Whispering_galleryReally!These are some useful links to different sections of the HyperPhysics website that provide information on harmonics, overtones, and so forth, for sure . . .
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/rescon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/music/otone.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/waves/opecol.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/waves/clocol.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/music/brassa.html#c1For sure! Helmholtz resonators are another fascinating aspect of sound, which is fabulous . . .
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NOTE: There are links to very detailed information regarding the acoustic physics of specific musical instruments at this website . . . ]
http://www.phys.unsw.edu.au/jw/Helmholtz.html[
NOTE: The resonant behaviors and characteristics of bottles and jars was known in ancient times, where auditoriums and theaters sometimes were lined with large wine bottles that were filled with varying amounts of sand to change the acoustics of these listening rooms based on the expected audience, type of play or performance, and so forth and so on. In this respect, I had a bit of FUN when I was designing and building the RAE Multimedia sound isolation studio, which among other things has a fully-floated floor that sits on rubber mats made from ground truck tires; triple walls, ceiling, and floors with generous air spaces and heavy fiberglass insulation, as well as multiple layers of different thickness sheetrock (a.k.a., gypsum board); and a few carefully tuned Helmholtz resonating panels and so forth and so on, which makes it extraordinarily quiet with respect to external noises and subsonic vibrations . . . ]
http://en.wikipedia.org/wiki/Helmholtz_resonanceFabulous! 
P. S. One of the most simple and effective ways to remove the bass and midrange "boominess" of a listening room is to get a few rolls of fiberglass insulation, which you stack in the corners of the room. If you prefer, you can cover the rolls of fiberglass insulation with a cloth screen or whatever, which makes it look a bit more stylish, but regardless of how it looks, it works very nicely and does
not require a lot of work . . .