Front brakes grinding
 

So here is my issue. Took a 3 hour road trip down 95 today. On the way home we hit a lot of traffic. Stop and go for a bit, then we would get up to speed for a few minutes then traffic started braking hard. This went on for about an hour. Suddenly my front brakes started grinding when I'd brake hard. I just replaced the brakes and rotors all around in April and this is the first time I've driven hard since. Mainly just drive it to work and around town so braking has been normal. Coincidentally just before the grinding I heard a pop under the truck like a rock hit the under carriage. When I got home the grinding isn't there when braking normally but again when I brake hard it comes back. I put it in reverse and brake and they seem to stop until I brake hard again. No smell coming fr brakes or pulling when I brake. Any ideas?