There's several things to consider here!! First, it's important to understand the difference between a chuckle & a grunt!! Chuckles are used by bulls to call in other or stray cows. They're also used to keep track of the cows within their herd. Grunts are not used for cows, they're & can be challenging or intimidating, which are reserved for bulls, especially bulls threatning anothers bulls right to either keep or have the right to breed his own cows.
When that's understood & you're familiar with both sounds and can tell the difference, the choice of needed sound for the particular encounter becomes much easier & most of all makes sense to the real bull, thus avoiding raising a red flag in his eyes!
In your case, if you made contact with a bugle and a bull responded with chuckles only, he then reacted to your bugle by trying to gather or round up his cows so he knew were they all were, because of an intruder in the area. If you would have cow called and he chuckled then he simply would be calling you to him not using grunts where intimidation would be involved.
Too, this bull when he chuckled he was talking to his cows because of you!! But he wasn't responding to you, but because of you, a possible threat!
If when you bugled he grunted only, which I may add is very rare, a grunt or series of grunts is normally followed by a screaming bugle. Then he would be telling you, you're not welcome move on or else!!
In most cases when a bull quits calling back, he's on his way in. Most hunters just don't stay put long enough to allow him to show up!! Instead they leave within 10min. thinking he's left!! This is espescially true when working a bull with cow calls.
This can also happen with bugling if you don't overdue the calling!!
So what should you do???? If a bull chuckles you, move in as close as you dare or whatever the terrain allows & give him what he wants, another cow, no need to overdue it, let him know you're there and available, when calling there's a good chance he won't say anything as to not give away his position, to the bull that he just heard further back, he'll just show-up & swipe this cow, so be patient, wait & let it play out!!! Remember, if all goes quiet he's coming, even a couple pleading cow calls could speed him up a little, but don't over-due it!!!
If you bugle & and get an intimidatinmg response, & you're very close like you said, (100yds) then don't bugle anymore, no need to!!! Move in once again as close as possible, (always watch the wind) once there start giving pants, start slow & rapidly increase the pace then flow right into huffing style low keyed but rapid chuckles, this shows you're trying to call a cow or cows to you & you've smelled them or made eye contact, you're showing excitement in your find. This is what a real intruder bull would do, especially a sneaky one!!! The herd bull should come unglued, (very vocal) & be on his way in, he won't let you steal a cow right before his minds eye.
If he screams a challenge & is just out of sight, throw in disteressed cow sounds, they're painfull & panicky sounding & very pitchy, along with low gutteral desperate growls with a lot of thrashing & stomping around, make it real!!!!! This shows the real bull you've got one of his cows but she's resisting his attempts to steal her. I call this "The Threat" You've posed a direct threat to his herd & he won't stand for it!!!! If he continues to scream & not advance, (which is highly unlikely) and there's two of you, leave the shooter there while the caller leaves the scene thrashing & calling all the while backing out as though he's forcing the cow away, the bull should show in seconds!!! I've had these methods work out many times!!!!
By understanding elk talk, you'll have many more close encounters & know how to handle them, raising few "red flags" therefore that translates into full freezers!!--<grin>
If you want to know what some of the sounds, like Pants, huffs & The Threat sound like, then go to our Site and click on Elk Sounds!!
Hope this helps!!!!---------------------ElkNut1
