I can only base my reply on experiences with one bun - so please don't take this as gospel.
Nibbles will cry/whimper very softly whens she's really scared (if I'm having to handle her in ways she doesn't like in order to trim nails, etc.).
She honks, oinks, grunts and grr-honks when she's happy, excited and wants to play.
My guess is that Peg's on target in her thoughts that Lily's possibly had some bad (scary, painful) experiences with grooming, and/or that she's scared that you might hurt her. (Not because of you, but because she's a rabbit and they're hard-wired to perceive restraint of any kind as a threatening, predator-type behavior.) And I'm sure the mats themselves can get very painful, pulling on delicate skin.
You probably already do this, but just in case... if Nibbles is scared or upset, I just talk to her in a low, soothing voice and tell her that everything's going to be OK, I love her, that I won't hurt her, etc. My feeling (after having her with me for close to 2 years) is that bunnies are smarter than we think they are, and that they understand more words than we might guess - which is why I now try to let her know (in words) that I would never knowingly hurt her. I also tell her that I'm sorry that I have to hold her still, but that whatever it is that I'm doing (example: nail clipping) is going to help her - in this case to be able to walk, hop and run better, to groom herself more easily, etc. I also do tell her that it is important for me to help her take care of her feet and nails. The thing that surprises me even now: she seems to understand that I'm trying to help her and is much, much more patient *when I remember to talk with her a bit beforehand and explain what I'm going to do.*
Sometimes I think that I'm just imagining all this... but I definitely *can* see a difference between the times I don't communicate about what I'm going to do, and the times that I do.
I know that Lily is very new to you (and vice versa) and that it takes time to build trust, but you might want to give this a go.
And I'm glad that she is so "talkative" - that can be a lot of fun!