to echo to this (customer has sent his board for inspection) I came to a similar issue on a specific NBv4 today which had also a little "chirp" sound just before the boot, like what the customer complained on his prizm4.
Both NBv4 and Prizmv5 got their boot timing adjust but that particular board sounded different.
The difference was the SD card (!). We (with TCSS) are working together to source the best cards but it seems we got recently some mix and match, not a lot but enough to question it. I first noticed this while replicating the sd cards (custom, home made binary SD card replicator) and some cards, while "working" (ie, you can read them on the computer, copy files, format) would have a worse benchmark compared to top ones. Those cards are usually slower to format and to copy files on it, they have a slightly slower reading speed as well and they are "noisier".
We got the same situation back in the day (256 MB SD cards, CF4 era or something, welcome to the time machine) and when getting samples from a new supplier, the Sd cards we tested were generating a lot of noise when being accessed by the core MCU. They were also using 2 or 3 times the normal current, which could be part of the interferences we got.
Much later, I watched "Bunnies" (famous xbox hacker) talk about SD card clones and I came to realize we are probably all getting "clones" of SD cards at some point and it's hard to know where they have been manufactured for real (you can for sure trust the made in taiwan/japan/china that is engraved, or the Sandisk brand, and have santa claus butter your toast in the morning).
When selecting a manufacturer / retailer, we (with TCSS) always ask for samples to test the quality but along the river, it seems that we are sometimes getting what we are getting.
The "weird" ones I got recently aren't as bad as the 256 MB from the past, but they can be slow and a tad noisy compare to the ones that perform normally. They will work perfectly in a saber board, but as I've seen, their slower reading speed make the boot access not matching the expected timing and I got a chirp.
On the same NB, I used another SD card and the chirp was gone.
so, Icesaber, I just hope you sent your SD along with the PRIZM board, so that I can diagnose this as well. It is possible that you got one of those slow cards. If so, I will replace it as well.