even when the car is stationary. what causes this? is there anyway to fix it?

It is probably just the radiator fan running.

When the engine is cold, check that there is water in the radiator tank. It sounds like your car is overheating, possibly due to lack of water/antifreeze in the radiator

l bought a clio and the previous owner told me that he had changed the engine mounts as the old ones were making noise. mabye you have the same problem. you could get it checked out.

Is it revving high according to tachometer (rev counter)?
Please give more info.