Alicey, as much as I also think very highly of SR (the entire Recommended Drive section is pretty much based on SR!!), I have to side with Seagate when it comes to noise measurements.
All the HDD makers use highly sophisticated noise testing facilities and they have standardized as an industry on SOUND POWER as their reference -- this is the most precise way of expressing noise. Seagate's own specs suggest that the V may be a bit noisier than the IV -- either that or they have moved to a slightly more conservative expression of their spec to accommodate for larger sample variances? In other words, maybe most 1 platter B-V still do 2 Bels (the 1-platter B-IV spec) but they now specify 2.4 Bels (1-platter B-V spec) because a few of them actually get to 2.4?
I have not had the chance yet to compare the 2 side by side (2 single or 2 double platter versions of the IV and V) but have read enough commentary by those who have to believe the difference is not very significant. In any case, if there IS a difference, Seagate themselves say the old one is quieter.
3.5 dBA, btw, represents an audible difference. A 1 dBA difference is only just audible, 2 usually, 3 more so. Most people hear 10 dBA as a double or halving of sound level. If you get caught up with power vs sould pressure level differences, this discussion could go on ad nauseum, so let not go there.
There are lots of other resources dedicated to just sound issues.