Ever since I installed my Meriah cooling fans I've had a very slight vibration that I could feel in the body of the car when they were running, especially when they were both running. I decided that something had to be slightly out of balance. I removed the fan blades from the motors and found that they were slightly out of balance.
I used a couple of small boxes, thin wood strips and a shaft that fit snugly through the blade hub as a balance jig. I was able to determine that the blade assembly was heaver on one side.
I used some stuck on thin lead used for balancing ceiling fans to balance the assembly. The balance process takes awhile and I used masking tape to stick the weight on and kept cutting it smaller until it was balanced.
It only took a very small amount of weight to balance them, and now they both run very smooth.