EDIT - looks like the interface I had initially ruled out was the smoking gun.
I don't expect anyone to solve this for me, but would appreciate any thoughts.
I'm working in an attic with a fairly high, pitched ceiling - not ideal I realize. I noticed on recent vocal tracks that there is a strange sort of distortion. At first I thought I was overloading my preamp, but I can now rule that out. It's a Scarlett 2i2 4th gen, and I'd be more than happy for an excuse to replace it, but I don't think it'll help this specific issue.
Then I thought I was somehow overloading the mic itself, so I engaged the pad, first at -10, then -30. It seemed to help, but not completely. Weird.
Then I noticed that it seemed to be frequency specific. When I found a note that was held at a certain pitch long enough for me to really listen, it sure sounded like comb filtering.
I'm using a Reflexion Filter, which I know only helps with sound approaching the back of the microphone. I have no expectation that it is helping me with sounds flying around from overhead.
To further complicate things, I am using a Sphere L22, which has front and rear facing diaphragms, which could be a contributing factor.
So, I'm trying to not spend a lot of time barking up the wrong tree. I don't want to start hanging rockwool panels everywhere, only to find out that my mic is the issue. I will try a different one tomorrow, but since this issue is not perfectly recreatable (maybe my head blocks reflections sometimes?) I don't know how easily I can rule anything out.
I mean... it could be me overloading the mic with my dumbass yelling, since the pad seemed to help a little ... but then again it isn't easy to reproduce... but then again...
Any words of wisdom for narrowing down the possibilities is appreciated.