Why is my steering wheel shaking at Idle and low revs?

The car is an automatic 2005 Ford Focus ZX4 SE. I noticed the issue around 100K miles, either during a cold start, or at Idle in any gear, the steering wheel vibrates/shakes violently. Any speeds in excess of 25 mph the shaking stops and becomes a deep hum, rather loud and dangerously soothing on long evening drives. the vehicles is now at 106,000 miles. The engines mounts have been replaced twice once at 50k and 100k miles and have are in like new condition, but for how long i don't know. The amount of shaking has neither increased or decreased. I haven't had much time to look into it, but i'm hoping there is a guide to be made of this.