I’ve experienced a clunk sound on my truck associated with the drum brakes before. It happened b/c the shoes weren’t sliding freely against the backing plate. They’ll stick then when they come unstuck they make a cluck sound. To fix it I had to lube the sliding surfaces on the backing plate up better with some brake grease. Something like that probably, or a spring has broke, shoes have come loose from their rivets, etc.
Removing brake drums is a common complain we hear here. Spraying with rust penetrant where-ever it makes sense as described above (including the adjuster through the back hole) and letting it soak for a couple days while you use the car helps. To finish the job, It helps if you can first back off the brake adjustment, so it’s worth taking some time to see if you can figure it out. How that’s done exactly varies from car to car. On my Corolla it’s a really difficult job if done when laying on the ground under a jacked-up and jack-standed car, b/c of the angle you need to get your head and eyes in to see the adjuster wheel. If the car was on a lift and your were standing up while you did it it would be much easier. On the Corolla I have to tie a makeshift piece of wire to something above to hold the little lever up then use a custom bent screwdriver to turn the wheel. On other cars there’s a hole right in the front surface of the brake drum so it is easy as pie to adjust the brake shoes with that design.
To remove the drum. I don’t recommend the hit it with a big hammer approach if that can be avoided. Too easy to damage something. Start with installing the correct size bolts into the threaded holes in the drum and see if you can push it off that way. That method is surprisingly effective btw, and I expect it will work for you, especially if you can back off the adjusters first. But even if you can’t, try it; I’m guessing the bolts will still probably push the drum off without much difficulty. If the drum remains stubborn, heat it up with a propane torch. This can take a while b/c the drum can store a lot of heat. So heat it up, then wait 15 minutes, and see if the bolts will push it off then. If it is still a no go, take it to a shop who has an acetylene torch, that’s will get the drum really hot and should get the job done then. I’d do that before whacking on the drum with a big hammer. If all else fails I guess you got nothing left but to whack it with a hammer, but do that as last resort. A few love taps with a big hammer using only moderate force wouldn’t do much harm likely, so that might be worth a try if the drum remains stubborn.
I’ve told this story here before, but years ago I was trying to remove a brake disc which was rusted stuck to the hub. Nothing I tried worked, so I heated it up with a propane torch. Still nothing. So I was laying under the car wondering what to try next, not even touching the disc, when the disc suddenly let go of its grip on the hub up and dropped on my leg … lol … that’s why I say heat combined w/patience often works.