How to identify a thought

I have a question about identifying the thought that is leading to a feeling of dread or guilt (I can’t figure out which). The feeling seems to always be there, bubbling below the surface, I just don’t always notice it. When I pay attention, it feels like a tightening in my chest. I am not sure if it is related to a specific thought because it seems to always be there, just a generalized feeling of dread. I am curious if you have any advice on finding the related thought?