1998 Ferrari 355 Spyder

Could also be the bypass valve.

 

Where is the noise coming from?
Left
Right
Front
rear
Undereath the rear wheel well

or, indeterminate?

 

+1

 

I had the same noise on mine, had it investigated and it was an air pump in the right rear of the engine room, It runs when the car first starts only, goes off when the car is warm. Does not go on again during the course of a days use.

 

The air pump does not make a rattling noise. It is more of a high pitch whine that goes away when your car is warm and it comes from the right rear of the car.

If it is a snapping noise it could actually be one of the belts. Sometimes they emit a snapping almost rattling noise until they warm up. I find they sound as though the noise is coming from the left side of the car.

Your clutch can make noise if it needs grease repacking but that usually stays whether hot or cold.

By pass valve should not be effected by hot or cold, in my experience.

 

Search the word 'hydrostatic' in the 355 section. It sounds like the common belt tensioner trait of making some noise before warmed up fully. Rifledriver explains this in an old post.