I just want to confirm, wind instruments and stringed instruments are completely incapable of creating an unpitched sound correct?
No, certainly many instruments that make pitched sounds can also make unpitched. Often this means using "extended techniques," i.e. doing "weird stuff" outside of normal practice, but some of these techniques are quite common. As a violinist, the first thing that occurs to me is what fiddlers and mandolin players refer to as a "chop," producing a short sharp unpitched sound by muting the vibration of the strings:
Many wind instruments can use an extended technique that simply "breathes" through the instrument rather than producing a pitch. Often this can be colored by the dimensions of the instrument and might be "slightly pitched":
There are also "pad slaps" which can be "mostly pitched," but quite percussive.
And of course just about any instrument can be struck in some way, even if not normally a percussion instrument. For flamenco guitarists, slapping the body of the instrument is hardly an "extended technique," but I've seen harp pieces calling for it, and done my fair share too.
The human voice is certainly a pitched instrument, but the entire field of "beatboxing" is evidence that it can produce percussive and unpitched sounds (just say "sh"!).
So if all this seems like "cheating," keep in mind that even when many of these instruments produce pitched sounds, these other sounds are mixed in. A violin note might start with a momentary sibilance as the string is set in motion, and a sung syllable starting with a consonant or sibilant could be dissected and found to start with an instant of white noise.
Edit: I thought of at least one wind instrument that is explicitly intended to make a very unpitched sound: the LP Wind Whistle (looks like sadly not being produced right now, though there are similar products).
Used effectively by the inimitable Andre Ferrari here:
It even got its own "solo" in the live version of "Shapons Vindaloo":
It may be played by a percussionist, and sold in the "percussion" section, but it's totally an aerophone in a Sachs-Hornbostel sense.
Not exactly. The different between "pitched" and "unpitched" sound is actually a matter of degree, not discrete. An instrument that is heard as pitched creates mostly the same frequency and frequencies within the harmonic series of the fundamental frequency. An instrument that is heard as "unpitched" creates more of a range or wash of frequencies, but in almost all circumstances, that range will have an amplitude peak at a particular pitch.
Timpani, for example, aren't fundamentally different from other drums like bass drums or tom toms except that they are engineered to have a "sharper" peak at a particular tuned frequency. Drumset players tune their drums as well; larger drums sound lower and smaller drums sound higher, generally speaking.
The more an instrument sounds like white noise, generally the less "pitched" it sounds; the snares on a snare drum is one example, and cymbals are another -- though the same relation between pitch and size applies there as well.
So that's the background. The more direct answer to your question is "no" -- electronic instruments like synthesizers can create white noise, or even the "least pitched" sound possible -- a completely even distribution of amplitude across the audible frequency spectrum. Many instruments can be played with what composers call "extended techniques" -- often using your (e.g.) violin or trumpet like a percussion instrument, or playing it in a nontraditional way in order to generate a novel sound.
Any stringed instrument can also become a percussion instrument. Hitting the instrument body is very obvious - but just with the strings, you have more options too.
For starters, you can have the strings hit the frets. This divides into "slapping" where you hit the top of the string and push it into the frets; and "popping" where you pluck the string outwards far/hard enough so that it hits the frets on the way back. These are popular techniques in bass guitar, especially in funk where the bass is possibly the most important part of the rhythm section. They're also used in regular guitar playing though, sometimes in the same way as bass guitar, but also in other techniques such as "chicken picking" in country music.
If the strings hit a well-defined fret, or are held fretted as they are slapped/popped, then the result can have a pitched component as well. This isn't always the case though, and even when it is, the unpitched element is a major feature.
And then there's the noise that the pick (or fingertip) makes when it hits the strings. If you mute the strings, all you're left with is this noise. That's used very widely with electric guitar - add a wah-wah pedal, and you've got your classic "wackawacka" funk sound.
But even without that, the pick noise is a thing. With acoustic guitar backing other instruments, very often the pitched element of the guitar can get lost in the mix, and what you mostly hear is the strumming noise of the pick on the strings. You can make this more or less prominent with different gauges of pick - a lighter pick will give you less volume from the strings, but more noise from the pick as it flicks over the (stiffer) strings.