Everyone has done a good job describing them so far.
When I speak, I use a lot of sound effects. (Yes, just one of my many irritating qualities.)
Rumbly BRROMMMUMUMUMBRMOOMMMM MMMMM
Buzzy zzzzz -tink tink tink tink -zzzzzz
the Harley comparison. Perfect.
Rumbly vibrations are picthed in a low register and you can feel them in your chest and your throat, in a good way. Buzzy vibrations are pitched in a high register like an electric toothbrush, just skating over the surface.