If the note is played percussively (e.g. with a pick, as opposed to e.g. a violin bow), at somewhere around 30 nps the attack of the note itself will be heard as a low note.
Why? Simply because around 30 Hz (give or take, depending on person, age, ear conditions, etc.) is the lowest frequency audible by humans.
In other words, when the percussive attack of the note is repeated fast enough, at a frequency within human hearing range, you'll start to hear that sequence of attacks as a frequency of its own.
If somehow the frequency of the original note manages to still get out (1) you will then probably hear two notes: the frequency of the original note played, and a second low frequency, a low pitched buzz caused by the hitting of the notes, if that happens at a frequency within human hearing range, which starts at around 25-30 Hz. In fact, this low pitched frequency will be the dominant sound, and the frequency of the original note will only be partially heard under special circumstances (1).
Sometimes we hear a buzz from electrical appliances, caused by the AC frequency (50 or 60 Hz, depending on the country), this might be something similar to that, starting to be audible at around 25-30 Hz.
(1) It depends on the note you play and the physics if the instrument. If you play, for example, an E5 note on a guitar (High E string at 12th fret) 30 times a second with a pick, that E5 note may still be heard, albeit in a broken and irregular way, because its frequency is about 660 Hz, i.e. much higher than the 30 Hz frequency of the picking itself. So there may be some time, about 33 msec, between one picking motion an the next, for that E5 frequency to come out somehow. On the other hand, if you pick a low E note on the bass (E1 = 41 Hz) 30 times a second, there is not enough time between pickings for that string to vibrate at least a few times and produce a distinct frequency.
Bottom line, if you start hitting a note very fast, 30 times a second or more, then the frequency of that hitting becomes the note you hear.
