Failing water pumps in my experience have a rough worn bearing "howl" most prenounced at idle, not a metallic rattling noise at all..replacement is straightfoward

 

the water pump replacement was a success. the noise continued!

i spent another half hour poking and listening, it is the engine side plastic radiator shroud. i need to either shave it back a bit or find a way to tighten it back to the radiator. it seems to touch the viscous fan at times. makes sense with the coasting, downhill, non accelerating usual cause of noise, the engine probably shifts a millimeter or two on its worn mounts causing contact.

 

finally figured this out. the item popped out in the pic that holds the hard line was loose in its hole. just enough that it would only rattle under momentum changes for a bit then settle back in.