Metalic single click noise when accelerating

[2011GreySentraS]

Hello folks, I'm new here. I spent about 3 hours searching the Web and have now been able to find other posts describing my issue. I have a 2011 SentraS with 99k miles on it. It has been fairly reliable, but it recently started to make a click sound when accelerating. At first it was once a week, but now it is about 10 times per 100 miles or so. It occurs when accelerating slowly from a stop or when accelerating at aound 30-40mph after taking my foot off the gas. The sound is similar to the sound of a stone being thrown up into the wheelwell, but more metalic and from the middle of the front middle of the car It is quiet enought that I dont usually hear it with the radio on. I had the transmission fluid changed with no change to the noise. My mechanic has driven the car but not heard the noise. The car accelerates and drives normally.
Any ideas?
Thanks,
Pete