[amsat-bb] AO 91 Rumbling Noise

Dear Folks:

Forgive me if someone has already discussed this, but I noticed a "rumbling" noise in the background during this morning's AO 91 pass.  What is that?  It almost sounds like a train is passing by.  It was not dependent on someone transmitting as it was present even while there was no one speaking.  But, it faded away as soon as the pass was over at my QTH.

73 de Bernie, KF0QS