Curious exhaust question

[87ChevyR10]

Just wondering, what creates that off-throttle raspy sound? I've listened to similar trucks to mine with long-tube headers and glasspacks and most seem to have that off-throttle rasp to them. My truck has long-tube headers and Purple Hornies glasspacks w/o extension tubing; so, it dumps right under cab/bed and mine does not produce that raspy sound unless I just don't hear from inside the truck.

Here is a video of the off-throttle rasp I am talking about: https://www.youtube.com/watch?v=r6ncodmonD8

Thanks!