Weird Vibe

[Rbratby]

I was on my way home today. I picked up my kids at the babysitters. On my way from the babysitters to my house. I was sitting at a light getting ready to make a left turn. Traffic cleared so I made the turn. I was in second at about 2500rpm and shifted to 3rd. As soon as I shifted into 3rd I got a weird vibration in the floor board. I stopped on a side road. I put the truck into neutral and increased the rpm. At about 1200-1300 the vibration started again. It feels like the exhaust is vibrating on the frame. I checked the exhaust and that's not it. I also looked at other places on the frame and nothing is hitting. Anyone have any ideas on what this could be? Btw I installed a sprung hub DD about 1200 miles ago. I also installed a 150 Raptor pump today. I left all the stock fuel lines and lift pump assembly in place. I turned the pump on with the truck off and it didn't vibrate the floor like this either. Thanks