No maintenance done lately. It just doesn't sound like an aspirated noise... really more percussive, like something slapping inside the case. But I'll explore your ideas, thanks-
Okay, engine-sleuths-
I'm getting the oddest noise... it sounds like someone is slapping the inside of the engine case once in a while with a wet towel, occasionally with a squeak like it might be a rubber towel.
Never at rest, not at startup (normal clackety-clack from CCT quiets down...