Fan in Macbook Pro started making a "purring" sound at times

Sounds like the fan bearing may be starting to fail. This can be intermittent and only noticeable when the fan is at a certain speed. A faster or slower speed may not have this bearing noise. I'm guessing this is what you are hearing.

You can take the laptop back to the techs to have them replace the faulty fan. It should be easy for a tech to figure out which fan has the bad bearings since all it takes is getting the fan to run at different speeds until the sound is heard, then figure out if it is the left or right fan. Sometimes it is possible to tell by physically examining the fan, but much better to identify it by listening for the sound by getting the fan to change speeds.

Hi,

I think you don't need to worry about the fan noise if the Genius's Diagnostics didn't detects any problem.