Sounds in drain pipes...tell me I've overthinking this

Replaced drain pipes in basement last year, now covered in concrete. Had a lot of rain recently, and heard distinct sound of water trickling come up from waste pipes. Are there alternative explanations than that water is leaking into waste pipes? We have French drains (internal) and one hears slow dripping in basement walls after rain, but this was the first time I heard sustained trickling from drain pipes. Could the sound be coming from the sewer (about 50' away)?

For further clarification, there is currently no trap on the pipe I heard the trickling from, due to work being done....

sound

There are so many ways a trickling sound could occur that we could not even list them. Or guess where yours is coming from.