Just a thought but you might want to take the plastic panel off below the dash, the one that gives you access to the pollen filter, when you change it and see if there anything in there that looks suspicious . You could also take the glove box out to give you more access to the area where the noise is coming from. If nothing is obvious I'll have my wife drive down a somewhat bumpy road and scrutinize the problem area visually I'd also push and pull on some of the plastic pieces up in there to see if the noise stops.

I had an annoying creak in my dash that turned out to be from the contact the radio made with a part of the plastic bracket that it rested on, problem was cured with some two sided tape and duct tape.

Good luck tracking it down.