[jluetjen]

In addition to up and down vibration, the engine also has rocking forces side to side from the crown-wheel pinion which are predominantly taken up by the transmission mounts, but I could picture the front of the engine having some movement. The shims would seem to help to ensure isolation of the mount from the chassis.

I'm pretty sure that I've got a bad engine mount in my car too and I suspect that under load the engine is rocking side-to-side because I can hear a rumble when the engine is under load above 4K rpm, but only in top gear. What is different between 5th gear and the others? The total load on the engine. A sudden lift-off of the throttle at that speed will also result in more rumble as (I suspect) the engine rocks back in the other direction on overrun.

Just my $0.02 of what is happening.