I'm driving over the engine plays in a loop of a 10 second pattern
of the rpms going up then down (with no regard for the actual movement of the tank).
I experienced the same thing when playing the Tiger. As Kyth has pointed out, SF modulates the sound for engine revs and there seem to be two sounds played, engine and pillars, with pillars being the track sound.
We have some Tiger sounds from the MMP mod for TvT (I hoped to add them all for the Tiger engine). I changed the engine sound to the tickover sound we have in MMP, though I had to slow the sound to half the original speed for it to sound good at idle and also added the pillars sound to the track sound we have. I can now drive the Tiger in manual and change gear according the the sound/engine pitch.
On the downside all the wing men also use those sounds, so the Tiger sounds a bit thin now compared to the faked sounds I had originally. A trade off I am happy to make as I like to drive with a manual gearbox.