Pet rats vibrateing

Question
I have 2 female rats that I adopted when they were about a year old.  I have had them for a few months now.

My one rat, Lola, has a problem with runny eyes and I was wondering if I can give her human eye drops or what should be done about this.

Also both of them will vibrate when I have them out of the cage cuddling them.  They don't do it the whole time, it is kind of on and off.  I know they are not afraid of me because they come to the door of the cage when I open it, they are very affectionate and if the are startled while out on the couch they run right to me.  So why are they shaking/vibrating when I am holding them?

Answer
Runny eyes how? Is the discharge red, yellow, green, clear?

The vibrating is very, very normal! If you time it, they probably do this every 4 or so days. A rat in estrus ("heat") displays by running forward a few steps, stopping short with her back arched and her head up, ears and body vibrating. It's natural, it's adorable, and it's a little bit sexual. It passes in about 12 hours and they go back to being their normal, non-vibrate-y selves. :)