You've got a mouse with a snare drum living under the bonnet.

If you open the air filter box, you'll find that's where they like to build their sound studios. Entice it out with cheese. To keep it from coming back, encourage a cat to live in there instead.

Yes, that's a really silly answer. But it's not really a sensible question. Too little information for someone to be able to help out. It could be anything.