I hit a dead dear and soon after started feeling and hearing loud banging or knocking sound from beneath the car specially in middle of car, mainly does it after like 10 minutes of driving or almost hitting 2,000 RPM, but not all the time. what could it be, took it to a shop but they didn't find anything.
Look for a loose plastic panel somewhere under the vehicle. If the fasteners that hold the front edge of the panel have been ripped out, the panel can randomly catch air and start banging around until you slow down. The banging only happens after you exceed some threshold speed. To repair, replace the fasteners, possibly with extra large washers to cover any area damaged by ripping the fasteners out.
This sounds like the wheel arch liner splash guard also known as mud guard. The plastic rivet seems to be knocked out, making the guard loose and hit on the wheel when driving due to air, but when hoisted they take the normal position, that would explain why the couldnt find the issue.
I would check if you have missing plastic rivets there