I know its a long shot because its very vague information but my car has suddenly decided the start rattling and there seems to be a squeak on one of the belts, the rattling is more like the normal idle engine noise in the cold but considerably louder, its a diesel and my first thought was that if i run it a little then it should subside when the car heats up but that is not the case, like i said i know its not much to go off but I dont want to panic and get it in a garage and pay through the nose if it might be something that will go away when run more.
especially since we do not know what kind of vehicle it is. and a sound clip might help. with the limited info… the belt might need to be changed, the idler pulley and belt tensioner could be making the noise.
Don’t know what the sound is, you might try a mechanics stethoscope and narrow down the location.
And if you hear a rattling sound from the engine, and you don’t know what it is, the last thing you Want to do is run it for any length of time or you really could be paying out the nose.