far too vague. could be anything, stone trapped in brakes, worn out pads, turbo, sooo many things. usually people have some idea though to be able to point in a general direction, like it was from the back of the car. the thing is, it could be very dangerous yet you just drive on hoping it will go away.
cars are expensive to run, especially old ones like yours, which seems to have needed some routine maintenance you are unhappy with, and now needs a bit more. in its day it was an expensive luxury car, and still is. mine is the same, but i fix mine myself because thats what i do for a living. you will just have to give in and take it to a garage i'm afraid.