Thanks for the money, pshep!
OK, first thing, I'd make sure to tighten down all the speaker screws then test it.
Millions of low frequency vibrations would tend to loosen the screws in my sorry ass, too. Just snug `em down.
Ok, I checked all the speaker screws and they are fine. I re-checked speaker cables and instrument cables. However, upon a closer look, the same thing is going on with the 115... and it is happening with each note... a slight buzz that seems to fade out. Could it be the pots on my bass?