I noticed vs I have noticed [duplicate]

In principle, "I noticed" stresses the act of noticing and doesn't necessarily imply that the result of your noticing has a direct effect on the present. Maybe you noticed something a long time ago and it is no longer relevant. Or maybe you noticed something recently, but can't remember what it was. Or maybe it is still relevant, but you just want to stress the act of noticing rather than the effect.

By the same principle, "I have noticed" stresses the effect of noticing. You now know that something is the case because you (have) noticed it earlier, i.e. started to know it. Also, this didn't happen too long ago.

However, I think in practice we tend to use "I noticed" more than the above discussion would suggest. For me as a native German speaker this is particularly obvious. It seems to me that in English the default for this specific thought (when you don't want to think about what you want to stress) is simple past, whereas in German it is definitely the German analogue of present perfect. I guess this is an example of the well known principle that English prefers simple past for verbs of perception.