Lately, he's added this angry-sounding grunt that seems like a scream except that it's not loud, like he lost his voice but otherwise it would be eardrum-piercing.
Anyone else see this in their kid?
It's hard to say without "seeing" the behavior firsthand, but I'm assuming its related to his limited verbal communication based on your post.
For instance, at 16 months, DS (who was totally non-verbal then) would get frustrated and throw himself on the floor, flail a little bit, and hold his breath until he turned red (instead of screaming). He mainly did this when he was frustrated about being unable to communicate something.
For the record, most non-verbal kids can scream just fine (even if they don't talk at all) but do tend to express their frustration physically before they get to the vocalizing stage.
Glad to hear he knows some ASL. The inability to communicate is VERY frustrating for toddlers.