I don't remember how these particular fans react to the reduced voltage, but in general they spec the voltage range based on the whole line of fans... the models starting out with the lowest RPM won't work at 5V, but the higher RPM models will run at 5V. In other words, with most fans (ignoring the really fancy 120mm & larger that have multiple zener diodes inside) if they still have a fairly high RPM at 7V, they'd be able to run at 5V, so the odds are that [whichever] you choose you'd be able to get the RPM down to a reasonable noise level, though for complete silence you do need be more precise in fan selection.