Knocking Sound After Breaking?

This is an issue for a friend’s truck. He explains that whenever he brakes and comes to a full stop, there’s a knocking noise coming from the front end of his vehicle, from inside the engine bay specifically. It’s not a knock as soon as he brakes, he says that he brakes, comes to a full a stop, then a couple seconds AFTER, the knocking noise can be heard. He also mentioned that he replaced three motor mounts about a month ago but that didn’t fix the issue, he was also told by a garage mechanic that it could potentially be a transmission issue or maybe a CV axle issue