Why does my vw bora 1.9 tdi sound like a tractor?

Why does my vw bora 1.9 tdi sound like a tractor? Sometimes it did sound ok, it would be noisy at first but then clear but lately it is like there is a little man inside the bonnet hammering away and it wont clear, there is also black smoke coming out of the exhaust when the noise is there
I took it to 2 garages who both charged me £130 to say that the only problem according to the machine was a sensor, that my car dosent even have
I am really angry at the moment as i feel that some mechanics take advantage of a women who fetches a car in
Does anyone have any idea what could be wrong, how much it will cost to repair and a decent place i can take it? The 2 garages i took it to werent sure what was wrong but wanted me to spend £300 to see if it was the turbo, though they really werent sure
Please help
Nicola