I have the GTS aftermarket headers with GTS mufflers. I have a chirp sound that sounds very similar to yours in my LH Muffler. 🐣🐣🐣
I accept it as a quirk, one of many, that a car will gather during a long lifetime.
I do think it is something that has broken a weld and likes to rattle or resonate at just the perfect pitch on occasion. Whether it is worth opening the muffler top and effecting a repair would depend on whether it annoys you that much.
Larry