Ford Fiesta hunting, revs high and mis firing?

When returing from work in my ford fiesta, the EAC FAIL started to display and my car started to rev high and lose power and appeared to stall even though i was driving perfectly fine. I knew then something was wrong, i pulled over and called the AA.When the engineer came out he pluged his p.c. in and went through the spark plugs, leads,battery,After testing all these's areas.He issued me slip stating possible throuttle fault? The next following day i booked it in the garage and they ran there test on the car.The results were ignition coil fault. They replaced this and after driving the vehicle it appeared fine, revs back to normal, no hunting. But i can still hear when reving the car sometimes, a puff kind of noise coming from the exhaust? Any ideas what this could be? When they garage changed the ignition coil box, they said it was wet, water had some how got into it? How can this happen, also my spark plugs had rusted too, due to water getting inside them? Anybody know how this could have occured or have experienced the same propblems with there car? Please let me know.

Thank you.