Because of the bicycle wheel sound I vote for damaged or poor bearings in the turbo. That might let the blades contact with metal periodically even. Or the intermittent noise might be bearing related only in the turbo Of course the seals could trash themselves somewhat if that was going on. Just pulling the boot and inspecting for oil might do it one way or another.
Most internal noises in the block would tend to be more constant in my opinion but then there are exceptions. For what it's worth and probably not worth much. Also valve seals do not in any way that I can think of create a noise intermittent or otherwise regardless of their condition.