I have seen this on a couple of cars in the past. I think it is a venting issue. Check that your lines are right. You could try driving with the cap loose and see if it is gone before looking further. Could be something as simple as the cap. I used to have an Olds 442 that did this, swapped cap and it went away. Long term answer on the Olds, turned out the lines weren’t hooked up correctly for the tank venting.