Stevs, thanks so much. Yep, this happens mainly on freeway driving when I let my foot of the accelerator. I was thinking the serpentine belt but it kind of makes sense it might be the water pump considering switching off the ignition stops the noise and the water pump is electronic, kind of resets. I am still on the original pump.

Will look at changing both the belt and pump in a couple weeks as I have a long trip in August out of town.

Folks, this might just be symptom to watch out for so you don't get stranded with a busted water pump.