My boyfriend doesn't like it when I say "yeah"

I'm not trying to get any sympathy or anything, just advice for what else I could say instead of "yeah."

Today my boyfriend was very angry and he started going off about it. I was saying "yeah" as he was explaining (and I do this often) and he had gotten to the point of anger where he directed it at me and insulted me. I immediately told him that wasn't cool and that we were done for the night. After a little bit, he apologized and told me he needed some time to cool off.

The thing is, it's just kind of a habit for me to say that. He's communicated to me before that he hates it when I say it because it makes him feel as if I'm not actually listening. And I'm listening, always.

I act differently when I'm not listening, trust me. But in a way perhaps I'm just not relating to what he's saying? I say "yeah" for a lack of better words. I can't relate to his anger (I rarely get angry and when I am I handle it differently than he does) but I can sympathize with it.

It's the equivalent of nodding and mhm-ing during conversation if this is hard to grasp.

I feel like I should apologize for it because I haven't made a real effort to stop (though I do correct myself every now and then). I just haven't apologized yet.

I don't know. I just would like advice for how I can show him that I'm more engaged. Like other ways to communicate that I'm listening to what he's saying.