Personally, unless you can see something obivous rattling at idle in the engine bay or under the car, I wouldnt bother looking into all these suggestions unless you know what you are doing.
I'm on about the clutch - Just suggested checking fluid levels under the bonnet - oil level and clutch/brake fluid. The most basic of vehicle maintenance checks
But brake fluid and oil levels shoud be checked weekly anyway as part of a normal basic car maintenance schedule.
Imho, its best to check break fluid every time you lift the bonnet, as it takes less then 30 seconds but can mean the difference between life and death.
You would notice if your brake fluid was low from the amount of travel on the pedal, trust me. And if its checked weekly, like it should be, then it will be fine unless you have a leak, in which case, you will know for sure you are low on brake fluid.
At the end of the day though, its shooting in the dark if the OP hasnt got a clue, the chances of it being the brake fluid (which I hope he has checked when he has opened the bonnet anyway) are probably 1 in 99, there has already been loads of suggestions posted here, and there are many more possibilities. Its not really something that is easily diagnosed without seeing the car/problem itself.
Id just recommend get to Olympic and see what they say. As I say, looking round for the problem if you dont really understand what you are looking for can take hours and you still may have no idea. A garage/mechanic will hopefully be able to diagnose the problem without much hassle, and far quicker providing its not something thats blatently obvious.