I would check the line and where it is routing next to before the pump.

recent exhaust change?

I ran my fuel lines and stuff first then had exhaust put in.
had an electric pump along the frame rail infront of the wheel well.

genius exhaust guy routed the new exhaust as close as possible without touching.

could be something similar since it only happens when hot. could also be after the pump if the line is too close to the exhaust or block.