Add a strip of felt inside between the spring and the casing.

Don't know how to isolate the culprit(s). But I would do them all.

I've heard people mention cramming a cotton ball through the spring...

I've got my preferred methods... cotton balls is the oldest trick in the book, however, the little cotton fibers will eventually get wrapped around the threads on the T-rods and makes a mess. So, I usually use kitchen sponges , or better yet foam, like you can buy at a dollar store in the kitchen dept.. just cut a little piece and pack it in there... don't worry about what it looks like , nobody will see it. when all the lugs are re-installed, before you put the heads on, pick up the shell and give it a good knock with your fist as you throw it up in the air slightly so neither hand is actually holding the drum and listen for any you may have missed. You will be amazed at how well this works. Don't forget to do this over the bed or sofa, unless you fumble your new work. Newer lugs have no springs.

Good Luck.

Glen

All of my Slingerland drums (1950s & 1970s) came from the factory with felt in the lugs.

I've always used a 1/2 cotton ball, flattened-out and tucked AROUND the spring, not in it. Always works, really cheap, no problems.

Synthetic foam will break down over time and make a gummy mess. Felt is a good choice as well, but nothing soft should be touching the shell.

Thanks all. I used polyester chair stuffing (similar to cotton). All's quiet now...

Why shouldn't anything soft touch the shell?

Cheers