Raspy noise from passenger side under acceleration - 1.0L Ecoboost

[JamesO89]

Some time over the last few months a noise appeared when accelerating in my Fiesta. Only happens when actually driving in gear (so cannot replicate when parked up, making finding the source incredibly difficult). The noise sounds like a tiny ratchet sound or like one of those old school wooden football clackers. Fairly quiet though. 
It is most noticeable in 2nd gear, does not increase in pitch as the revs go up, does decrease in pitch if lifting off the accelerator but then stops a couple of seconds later. 

Does anyone else notice this noise?

It doesn't seem to affect performance at all, just had it serviced a couple of weeks ago. (asked mechanic about it, he couldn't hear it)
It's driving me insane and I really want to know it's nothing that could get worse.