Purring/rattling sound from Mazda Miata?

Hello,

I'm stumped here: I have a 2004 Mazdaspeed Miata that suddenly started making a purring/rattling sound from the engine/transmission area. It's worst at idle, but I can hear it at all speeds including stopped, in all gears and in neutral, with the clutch in or out - it makes the noise basically all the time and gets more severe the lower the RPM. No other indication that there's anything wrong except the sound. It started on the highway a couple days ago and has gotten worse. It sounds like something is loose, like 2 large metal parts bumping together rapidly, with an occasional high-pitched squeak. Could this be a clutch-related bearing like the throw-out bearing going bad? Could it be the turbo? I can provide more specific details if needed, just looking for the best direction to start with.

Thanks!