You state that the idle is about 400rpm. If you are not making much oil press, and at 400rpm you are not, then the chain tensioner shoe will not extend enough, leading to.......
The timing chain guides being worn erratically, which causes the rattle when the oil press is low.
Short term fix, is to get the idle stable at 800-1000rpm when warm, and double check your oil level.
Long term fix.....replace the timing set (chain, guides, tensioner, etc.)