GREEN BAY, Wis., July 24 (UPI) -- An elderly Wisconsin couple said they feel like everyone thinks they are crazy because no one but them hears a persistent low rumbling in their home.

Bob Ehrfurth, 75, and his wife Leona, 76, said they are fed up with annoying, motor-like sounds and vibrations that rumble through their home, keeping them awake at all hours of the night.

The couple said they have no idea where the sound is coming from and have a hard time getting other people to hear it because it frequently stops, the Green Bay (Wis.) Press-Gazette reported Wednesday.

"It's like there's a semi parked right outside with the engine running, but when you look out, there isn't one," Leona Ehrfurth said.

The couple said city authorities, members of the media and acoustic experts have heard nothing when observing the home.