NPR really does have a signature sound. Here is an article about it. Top audio engineer explains NPR’s signature sound
Excerpt
“It really comes down to the U87 (microphone) with the bass rolled off.
“The U87 and most higher-end microphones have two switches on the back. One is a polar pattern, which is the direction of the microphone, and the other one is for the bass roll-off. When the bass is rolled off, you can’t hear the lower frequencies of my voice. The microphone itself takes them away.

The reason NPR came to this standard — and this was decades ago — was because most of our listeners are consuming in an automobile or with something else in the background. Back in the day, and even to some degree now, you roll down those windows and hear those low rumbling frequencies. We wanted our voices to get above that so that they could be clear, open and understandable to improve our storytelling.”

I don’t know why that causes and echo but it might explain why there are differences between stations.

You can try playing the stations on a digital format. Have you tried that? With a tablet or phone in the car? I don’t think I even own a regular radio anymore. :/ Just go to this website, write in your zip code and you’ve got all your local public stations. https://www.npr.org/stations/

Thank you both! About three days ago it stopped. I didn’t do anything to the radio, all the stations just stopped echoing.