Intermittent THUMP in radio

I have a 1996 Bonneville SSEi and have a problem with an intermittent thumping sound from the speakers. I have tried to isolate a common condition when this occurs but have not been successful. When is happens it comes in sets of 3 with a small break then another set. The number of sets is not consistent, maybe 2 or 3, maybe 7 or 8. Volume of the thumps is not controllable with radio controls. Turning the gain up or down has no effect. This has been driving me crazy for quite some time, any help appreciated.
One wierd note. Last year we drove from Indiana out through the Badlands, into Wyoming, Colorado, and back to Indiana. The radio did not act up one time on this trip until we were back in Indiana, almost the instant we crossed the Wabash river, wierd huh?
Thanks,
Gary