One of my HDT's tends to make a groaning sound at certain frequencies and at the same time on certain songs. It has done this since day one. Any ideas?

It's like a fast grating, and is tuned to something. Most famous for doing it at the same time on each tune. "Old Man" live, by Neil Young with just a guitar, and at the lower frequencies which would bottom out at a low "E" on a guitar. My first thought was a loose PVC pipe "rattling" being amplified thriugh the cabinet, but sounds like a driver. I reversed cables and it reverted to the same side. Happens on other songs too, but always at the same time. It is tuned to a frequency. Any Ideas?