New Prius Owner - 2011 II - Maintenance Question 100k

Hi everyone,

New to the forum and new to Toyota. I finally caved and bought one after years of owning just American brand cars. So here is a little about the Prius I bought...

It is a 2011 Prius II that was purchased from a Toyota Dealership. The dealership told me it was a trade-in. It has 96,000 miles so I am quickly approaching the 100k mile marker. I have read a lot of the other threads here about the maintenance that should be done to the so here is what I am most likely going to be going.

Coolant Drain and Fill - This is actually on sale at a Toyota service department near me for $129.00 - I was just going to have them do it.
Transmission Drain & Fill- This I am not sure if I am going to do or if I am going to have them do it. Really it depends on the price.

Obviously, I will be changing the oil at 100k miles. I was wondering if any of you would have any other suggestions or anything I should be aware of with this year car. I am pretty handy and have done a lot of DIY projects in the past on American cars (such as clutches, etc).

Last questions - I am also noticing with this car when I go on a major highway (over 55mph) I hear a lot of noise in the cabin. The noise I hear which I can't figure out is almost like a bass-y whirling noise. At first I thought maybe it was the tires but for some reason I feel like it just doesn't sound right. I know its hard to figure out without hearing the noise but I figured you all maybe able to help!!

Any and all help is greatly appreciated!!