Engine and Transmission Tech

I know this is a long shot but hoping someone knows the answer to this.
On some carbureted V8 engines the exhaust note actually gets louder on decel (throttle at zero and coasting while in gear) similar to an engine brake. Unlike modern injected engines that can "pop" or rumble on decel depending on the tune - it's just a loud drone. Harley-Davidson motorcycles almost all make this noise as well.
From what I can find on the subject in discussion forums it's a known condition but the cause isn't discussed much, I did find a suggestion that it can be due to a leaky power valve but not much else. The discussions on Harleys are a bit more all-over-the-map on this - some say cam profiles, exhaust, etc. but none of the V8 discussions go there, so I'm assuming with V8s something else is happening.
Here's a video where the drone noise is very prominent:

[www.youtube.com]

It can be heard a few times starting at the 3:30 mark, but they could be just adding the effect in post-production - the first authentic recording of the drone can be heard on the dyno at 8:12 when they replaced the Qjet (that didn't make a decel noise on the first dyno pass) with a used CarbShop 650 double-pumper and the drone showed up right away. It gets clearer starting at the 10:00 mark and can be heard in every subsequent dyno pass they make. I remember hearing other V8 engines make this noise as well. With all the background noise in the dyno footage, it seems unlikely this was an effect added in post.

Any guesses?