It appears from the pic that the lid does not sit down well (air leak), and also that dent on the top left may have your drum out of round. Take a couple of hammers and try to push that dent back out. Try to make the top of the drum as round as possible. measure acroos in several sots and try to get it rounded out. You can usually bend it back into place by grabbing across the top and pulling it back towards you. You want it good and tight, no air leaks.
Thanks for the advice Mark, I will give it a good thumping this weekend.