i have a leak on drivers side footwell and its louder when being driven ??

I bought a 2001 FORD GALAXY LX TDI SILVER about 3 weeks ago second hand, the owner said he had noticed the mat on the drivers side being a bit wet but didnt know what it was....car has just passed MOT in sept and had nothing to advise on it.
Today i noticed the car was a lot louder than normal especially when pressing on the accelerator, and it sounds like a hollowish pipe kind of noise but its near the front of the car on the drivers side.
As i parked i dropped my ticket by my feet and realised there was literally a puddle by my feet!!!! so something is leaking into the footwell and now is making the car sound very loud when driven......
Anyone have any ideas...i am dreading taking this car into the garage and would like to be somewhat prepared for what they may say...im 24 weeks pregnant so VERY hormonal...i do not want to cry if they say a ridiculous ammount to fix it.lol.
I tried googling and most seem to think it could be the air con, but would that cause it to be loud when putting my foot down????
any ideas will be gratefully recieved.