Older rat recently started squeaking sometimes when pet and when grooming, any ideas?

I have an elderly male agouti rat I adopted last June, and renamed “Duncan.” He was nearing 2 years old and had been relinquished at the small animal shelter when his owners decided not to take him on their move to a new state. He’d initially been adopted from the shelter as a baby with his brother, but the brother had died and these people never got him another rat companion, so when they dropped him at the shelter he was alone and had been so for almost a year. His prospects of being adopted as a large, older, male un-neutered rat were slim-to-none, while his chance of being safely introduced to another rat mischief were also minimal. As a very large intact male, he was destined to live in a cage alone at the shelter as a “sanctuary rat,” so I adopted him to give him love during his “golden days,” understanding he would be a lone rat (which is generally frowned upon, but in his case is the safest option). He was very obviously traumatized from everything and was a bit “nippy” with me the first two days, but I took things slowly and let him acclimate to the new place, new cage, new play area. He didn’t even know what a hammock was or how to use one initially, he really didn’t seem to know how to properly behave as a “Rat.” I let him decide how much human attention he wanted and provided a huge amount of enrichment to try to help with the fact that he was a solo rat and in a completely new world. Surprisingly, he turned out to be an extremely affectionate boy who loves being pet and snuggling. Recently, though, after being a very quiet boy, he’s begun randomly squeaking out of nowhere when I pet him, and then he will groom that area and squeak a few more times. There doesn’t seem to be any pain involved, it’s more like he seems “annoyed.” Sometimes when under a blanket with me on the daybed he will give a few of these squeaks, and when he’s just grooming on his own he will sometimes utter a few squeaks as well. At other times he’s perfectly comfortable and content to be pet for an hour at a time, and seeks out affection just as he had before. Has anyone else experienced older rats becoming more vocal like this? His health seems surprisingly great (he’s had doctor wellness visits and she’s impressed by his health for his age), he’s got a very small amount of hind end degeneration, but so far only really affecting his tail (which he holds to one side). And for the first 5 months I had him he was not squeaky at all. Is he just becoming a “curmudgeon” as he ages, or is there more going on? I wonder if he wants help with grooming, and he’s squeaking in frustration, since all my other older rats always had companion rats who helped them stay groomed when they had trouble reaching all the spots that needed cleaning. But if anyone has another possible idea, please let me know. Again, no evidence at all of pain, and he still is very ambulatory, going up ramps and going to all levels of a DCN, no signs of difficulty eating, no tumors, no respiratory illness, no other weird behaviors. Thanks for any info you might have, much appreciated! (Duncan below, prior to his recent squeaking behavior, waiting on belly rubs, with little toe beans on display ❤)