I just installed a new chain. It's a modern KMC x8 EPT. Supposed to be super rust proof and impervious to salt and all that... The SunTour Cyclone long-cage I'm using here was cleaned and re-lubed a couple of months back. The freewheel (SunTour 14-34) also. Anyway, I shortened it and got everything just right, or so I thought... when I finally gave it a whirl, I found things were amiss somewhere. I'm getting an annoying kind of clipping sound, and it seems like it is coming from the bottom pulley. Admittedly, I took this apart, cleaned and related. I may have replaced it with another SunTour jockey wheel even. But they're the same sizes, correct?

https://youtu.be/dI36YqtxePI
I used an "8 speed" chain because 5 speed chains are difficult to procure. This small difference in width wouldn't likely be the issue. For one, these derailleurs are used with all sorts of freewheels.