About the noise, it can't be guts because mares don't slosh. Only geldings and stallions slosh. It doesn't seem to have anything to do with whether they are clean.
My personal theory is this. This is an inner chamber up there where the penis hides when it is fully retracted. This is kind of like a ring feeling chamber it hides behind. So I think the sloshing noise has something to do with having the penis tucked up behind that chamber. I have had geldings make the noise when I am trotting them under saddle. Not a lazy trot but an excited, energetic trot. So I think that noise happens when the penis is tucked all the way up and hiding. I kind of think of it like they are sucking air into that space or something!
Also, I should mention sheath cleaning and bean removal are two separate things. I cleaned the sheath for years, got all sorts of gunk out, but wasn't until a couple of years ago that someone showed me how to find and remove the bean.
So you can be faithfully cleaning the sheath and miss the bean. Or removed the bean and leave the sheath dirty. The two don't necissarily go together.
The bean is actually hidden in a pouch on the end of the penis. I usually try to check that when the geldings are dropped. Sheath cleaning I usually do as a separate procedure, since my boys typically don't drop when I am sheath cleaning.
I have had friends say their horse is clean because his penis looks clean. Um, no, the sheath can still be gunky and the horse can still have a bean. You don't know until you check those things.
Wow, I guess I have been doing this way too long!