This one is interesting to me because I can hear (and speak) the difference but it's something I would have never thought of on my own. I think, that the difference here isn't in the vowel `-a-` but more on how the first syllable `rat-(/ratt-)` is stressed in Ratte vs Rathaus.
Bare in mind I'm not a Linguist, but my take on this is that the word Ratte just flows out of my mouth under one breath. The second syllable is is made without needing to "stop my voice" if that makes sense… I just move my tongue up to the top of my mouth to make a small `-t-` sound. Ratte is pronounced very quick.
Rathaus, on the other hand, there is a something like a full stop (but not really a full and total silence) in between `Rat-` and `-Haus`. And for whatever reason, it seems like I pronounce the `-a-` in Rathaus longer than I do in Ratte.
r/iamverysmart