I just got two new fronts on my 6mt and they are non-directional. But my rears (larger size tires) are directional.

Is this okay? Because the car will rumble slightly and the tires make a lot of noise in the lower speed range.

Anyone have this set up and have this problem too?

 

As long as the two fronts are the same size and the two rears are the same size (and installed/rolling in the right direction) you should be fine

 

The mixed tread designs (assuming non matched brands/models also) could cause an upset or imbalance in handling from front to rear. Probability of issues would go up in poor weather such as rain. Only a couple tire models out there are specifically designed with dir/asym mixing, the vast majority are not.