Audio 'popping' noise with external sound card in Mountain Lion?

I'm running Mountain Lion (10.8.1) and I'm getting some weird high pitched helicopter noise coming out of my monitors every time I turn on my audio interface or connect it to my imac once the computer is in session (on/logged in). The only way to stop it is to make sure the interface is on and connected and restarting my mac.

The interface in question is a TASCAM US-1641. It has the latest drivers and firmware update.

This has only started ocurring since last week, and hasn't stopped since. I'm assuming it's probably a bug with Mountain Lion? Never had this issue in Snow Leopard (previous OS).

I've tried to sudo kill coreaudio, to no effect, then reset my NVRAM, which fixed the issue only for about an hour, then it returned, except only when audio was playing did it make the weird noise as well. After turning off/on/reconnecting the audio interface, the original problem was back again.

This is really irritating as I have to restart my computer every time I want to use my external speakers. Anyone got any ideas?