Baffling radio interference

Thanks for all the thoughtful replies. I really appreciate how everyone is trying to solve this problem for me. Yesterday, I had a thought. We have an RV garage that is long enough to keep our camper and the pickup, provided I park the pickup with the front end under the cabover of the camper. In order to make that work, I changed out the antenna from the OEM to a shorter mast, shortly after we bought the truck. It occurred to me that the antenna might be the problem. With engine running, I swapped the antenna masts and found that the noise on my favorite stations was the same with either mast. There was also no discernible difference in the quality of the reception. Just for fun, I hit the seek button on the radio. It quickly found a "local" station (remember, I said we live in a rural area), and the reception was perfect, with no noise! So, perhaps, I just need to find stations that are closer to home. Only problem is, I don't WANT to learn Spanish at this age!! :o