Is it sort of "pbbbht" distortion that seems to complement the program played through it?
I'd look at two things.
1. Try setting your latency up a notch or two and see if it's still a problem.
2. Use Windows System to take a look at IRQ and DMA channel sharing. If the UAD1 is on the same IRQ as something (or for that matter, your sound card), physically move them around in the bus until they aren't conflicting.