Preface - I have anxiety specific to my car breaking down which makes it hard for me to tell the difference between “normal” car noises and those that are indicative of a problem.
When my AC is on, periodically when my car is at idle there will be a brief shudder. I also notice it when I put my car in reverse. It will also occasionally vibrate a bit at idle with AC off, but I notice it most when my AC is. It is not frequent enough that I could take it to a mechanic and reproduce it. Usually it is after the car has been running for a bit (5-10 min).
I wouldn’t mind it so much but recently my engine misfired and died, ~60 mph. It got a new set of plugs and coils and now seems to run smoother. But if there is a problem that is indicative of a larger issue, I want to take care of it.
take it to a trusted mechanic for a complete look over and inspection. This could be motor mounts that are worn, a dirty throttle body like Tester suggested, or a vacuum leak of some kind. Some things can be figured out with out the symptoms being present.
It could be an overactive imagination, like you say. I had a late 70’s VW Rabbit years ago. Part of the routine maintenance was removing & checking the fuel injector spray pattern. First time I did this, heard a very high pitched noise from the injector when it sprayed. Ever after doing that job, I could hear the injectors’ high pitched tone as I drove, esp when accelerating. But I never heard it before doing that job, although it was certainly always there from day 1.
