Engine mount sound when engine turns off.

Hello all, I am going to keep investigating this and will add new info if I figure something more out. In the mean time I wanted to go ahead and post a thread. I recently rebuilt my engine and have since drove it several times. Due to this I have been very hyper sensitive on every sound and I've almost worked myself up into a proper panic. If I am stopping at a stop light, right as I fully stop, then the gas engine turns off, I can hear a rattle that sounds like it's coming from the engine mount. I've heard this sound on many occasions on two different prius but on my last trip breaking in the new engine I heard it doing it 3 times. This might be normal and I just don't remember clearly. The best way I can describe it is that I feel as though the sound has to be coming from the mount. It makes that distinct sound that thick rubber and heavy metal makes in this situation. I am starting to worry maybe I didn't mount my engine properly of something. Any ideas? Do any of yall notice this sound normally?