Vibration - some possible causes

I don't know if it's just me, but for some reason today I thought my car's steering wheel was vibrating more than normal. I would not really feel it in the car, just the steering wheel and a little bit on the pedals.

Is it normal to feel any vibration in the steering around 60mph? Or should it be completely smooth?

It snowed in the morning, so I thought maybe some snow got in the wheel well or tire, but the roads have been clear and I still got some vibration around 60mph, and sometimes at around 30mph too.

I know my rear flex disk is a little bit cracked, could that possibly cause some vibration? It's very minor. I also got motor and transmission mounts in recently, possibly they were not put in 100% tightly?

Any ideas would be welcome, I know vibrations are a common question and hard to diagnose!