I think you may have the hydraulic mounts instead of the round rubber mounts. When the hydraulic mount sag, the oil pan drops down close to a rubber bumper strip under the front of the engine. If the gap between the bumper and the pan is down to about 1/4", it's time for new mounts. I found this gap to be 1/4" on my 940 (same as a 740 in this case). I replaced both front mounts and the tranny mount for good measure. The vibration at idle became much smoother.

I bought the mounts from Rusty at RPR for about $45 each.