I love when people ask questions because others often have the same issue.
I have been having the same problem in my 1931 Victoria. It runs like a top, idles well, but I get that little "pop" when I let up on the accelerator.
Looking through my books this morning, most of them said it was the carburetor running too rich, and to check the float level. I rebuilt the carburetor about two months ago and everything was proper then...but they are always good for an unexpected surprise.
I was going to check it out and see what I could find this morning, but got sidetracked and ended up taking apart a transmission that was on my to-do list.
I plan on getting to the "popping" issue sometime this week.