sock monkey
I was told long ago that PCI audio cards are very susceptible to internal noise. Might be why we have moved on. Check out Motu
Though I'm not a fan of A/D D/A converters being inside the PC, some designs (Lynx) are much better than others.
Lower cost interfaces with onboard A/D D/A typically see a noise-floor of ~-100dB.
That's not horrible... but it's not stellar.
The noise the OP is talking about wouldn't be the noise-floor.
The most probable cause is the Z97 motherboard using a bridged PCI slot.