2005 Ford Fiesta i'm going to sell this car if i can't stop this noise, please help

OK, over the last 6 months my car has developed a very annoying clicking noise, like electricity crackling (if you click your tongue on the roof of your mouth quickly between your teeth you get a similar sound)from around the cam belt area, drivers side, it started to happen quietly only on acceleration for about a mile, it has become louder & more prevalent over time. It mainly only happens when accelerating & can be heard as just as loud outside the car, enough that people turn to see what it is!!! It doesn't seem to happen when it is wet & it now happens for about 3 miles from start up, once the car is warm it stops, otherwise the car drives fine.
I have had it to my mechanic & he was stumped, very, very rare occurrence is that, his only suggestion was that it might be the water pump & to wait until it gives out.
It is due to be MOT'd next month & I don't want it to fail, I need it for work.
Please, please help.