Smoothing out the transition back to the Hum

Is there anyone with some experience who could explain the best method to achieve that perfect smooth transition from a swing/clash back to the hum?

I have a few fonts that I'm trying to fix, where the transition between the swing and the hum is very choppy and distinct. Thus far I've tried cutting off the first 3/4 second of hum and underlaying that with each swing and clash, hoping it would transition directly into the rest of the hum with no hiccups. But, that didn't work, unfortunately. It's as choppy as ever.

So what's the secret? The default Obsidian sound font and the ones that come with Obsidian v4 do not have this problem, nor do Novastar's sound fonts. How do I do this?