[Keith Seymore]

The "undulating" is called a "Boom/Beat" in the biz.

What that means is that you have two disturbances going on, very close in frequency, such that the sine waves gradually add and subtract, giving you this variation.

Anyone who has been in high school band has likely experienced this as they have attempted to tune their instruments.

In a vehicular context normally it is something like engine firing frequency coupled with a driveline vibration.

The good new is if you can fix one vibration then the boom/beat goes away; the bad news is then you have one vibration remaining - and - if it is engine firing you can't very well make that go away.

K