Doubt this will help, but my elderly Mk1 did this, due to the oil for the hydraulic tappets draining down while standing. I was told that the oil filter had to be the proper Mazda part which had a non-return device to keep oil in the system. Other filters may have had this but the genuine part was sure to have it and was no more expensive than most others.

Don't know if later cars are the same.

How long does the rattling sound last for? It's hard to diagnose sounds without actually hearing them, but as the previous poster said, it could be hydraulic tappets. However, I wouldn't have thought they would rattle after only an hour or so following a run.

However, you do have a couple of things on your side:

1) you've got a couple of years warranty to run

2) you've already raised the issue with the dealer, so you can't be accused of ignoring it. Do Mazda offer a 'quality assurance inspection' after 3 months of ownership to sort out any post-sale niggles? You could raise it again at that, or if Mazda don't do those, wait until the first service and raise it again.

No quite right to worry, what oil have you got in there and how long since changed?

I'd imagine an oil change and filter with 5-30 or 5-40, whichever is preferred, would lubricate hydraulic tappets. Synthetic oil.

However if it's a real old world big end rattle which it does sound like you'll need new big end shells, maybe new mains, and it might be cheaper to get a replacement engine.

Possibly oil pump wear, you'll only ever know if you take it to bits. If it's done it from day one just keep changing the oil and stop worrying.

Make sure you're using good oil filters with a valve that works. I think some let the oil leak back and you get a rattle.