2008 Volvo S40 2.

Normally tiney rattley noises are just heat shields that have come loose check under the bonnet and under the car for loose heat shields also the heat shield around the cat can corode and come lose hopevthis helps ivan

I left a note about catalytic coverter shields.
Woody

Are any of the speakers loose?

Sounds more like the water pump but who iam i to argue without my head under the bonnet

Sounds like you may have a exhaust loose or either the cat converter is stopping up. Some of these have baffles inside which will break loose and cause a rattling noise. When the engine is cold, start the vehicle and crawl under and hold the converter with your hand and even the exhaust to see if any pressure will stop the or alter the noise. Careful it don't take long for the exhaust to heat up. If these baffles are broke inside, it can cause the converter to become stopped up and rattle, this will also give an engine false info when it comes to the fuel and air ratio mixture. Not going to far into it, so i don't confuse you. Good Luck

Very hard to know without actually hearing the vehicle but,,,With what you describe,the cat/converter 'may' be making the sound.With the car off and engine cool,get under and give the cat/con a really give shove or hit.The insides of them do corrode and the internals make a very loud rattle.If this isn't the problem it will let you see if there is a loose exhaust mount that may also make a rattle.(The cat/con sound is like there is a very loose metal plate hitting something)Its actually the internals of the cat. that have broken loose and rattling inside it.Hope this helps

probb. dryed out leaves or needles in fanmotor right front under the dash take out fanmotor and clean fanblade.

is the spare tire hanging under there? i had my spare come loose and was banging around under there, maybe a shock has gone bad!

have they checked the sway bar stabilizers,or if a lower control arm bushing has moved out of place.or maybe a strut bushing and mount

I would check the shields that they install around the exhaust system the welds break and lets them rattle.