The vehicle has the SHOC engine, 180K miles and has had extremely reqular service since 70K miles.
The noise started " all of a sudden" while driving and it is very predictiable. Louder at idle and completely disappears above 2500 RPM and reorccurs coming back down at 2500 RPM. (The noise only exists at and below 2500 RPM and it's frequency increases / decreases with the engine speed) It sounds like a playing card in bicycle spokes only greatly amplified.
I think (a quess) it might be the timing belt slapping against the timing belt cover as might happen if the tensioner failed or is failing. The tensioner was replace with the timing belt, idler rollers, water pump etc. 40K miles ago.