After the toilet flushes, something in the wall starts making a noise indefinitely until I run the sink. The noise sounds like some sort of motor or compressor. It's very loud, like above conversation volume.
100% not a plumber but it might be an air lock in the water system? In which case i think you can remove it by turning all the taps on in a certain order to flush it out. But google since I can't remember which lol.