NOTICE: Q/A transfer from Vocaloid wiki's Mythbusters page
"Plucks" - are a faint metallic sounds, Kiyoteru, Gakupo, Prima, Tonio and Sonika have plucks in their voicebanks. However, in the case of Gakupo and Kiyoteru, the sound doesn't fully pluck and only the led up to a pluck is heard. These are a result of a glitch by the engine or a bad sample recording and appear between diphonetic data. With Japanese Vocaloids, these can often appear less noticeable due to there being far less diphonetics for Vocaloid to assemble in the first place. Low grade computers may throw this up more then higher grade computers.
Background tones - Sometimes when Vocaloids sing a background noise can be heard, like when Prima says a "g" by itself. The tone itself is similar to the sound of air blowing into a microphone gently. It is currently unknown what causes these.
Rasps - This was most noticeable in Vocaloid compared to Vocaloid 2. The engine will smooth out the samples, trying to hide the transactions between notes, the output result will retain a digital rasp. Vocaloid 2 has a more natural sound and less rasp to it although there is still a metallic tone.
Flatness - because Vocaloids are recorded in just one tone, they are often very flat and lack emotional emphasis. Tone can be added via filters outside of the software, although adjustments within the song itself will also aid in making the lyrics sound less flat. Append voicebanks are also another method of making a voicebank have some layer of tone without editing by switch them around as the song is played. SeeU and Miku are known to be fairly flat.
Pitching - Another issue is that some Vocaloid has issue relating to pitching. There is instability in some octaves in these certain Vocaloids. Pitching issues are present in a number of Vocaloids, however, the level varies with some being more stable then others. Miriam was the first well known case of pitching issue and was reported in Sound on sound magazine prior to her release, but she is not alone. Language is not a factor in this. The two most likely causes are either sample related issues (change of tone as recordings were made) or a glitch, although no confirmation has ever been said to be the cause of it. This issue, however, is easily resolved if a producer runs a finished Vocaloid wav. file through pitch correction software.