There seem to be quite a few people reporting this on XDA and I found some youtube videos showing the symptom that my phone (and its replacement) both have:

Nexus 5 Loud Vibration - YouTube

Nexus 5 Keyboard noisy and strong vibration - YouTube

I hear people saying this is "normal" but I have a friend with one that sounds like the quiet one in the 1st youtube link. I know the vibrator used is different than other phones, but I really find it hard to believe this buzzing/whirling noise is normal. 4.4.2 helped with the feel of the haptic feedback, but the noise of the vibrator is a hardware thing, right? No amount of software is going to make it quiet (short of disabling it entirely lol).

I didn't see a thread here about this, so I figured I'd see if anyone here has one that sounds like an engine revving up like that or if I've just been unlucky and gotten two defective devices!

 

Is it loud, too? Mine feels like it WOULD be strong if it were attached properly.

 

Ok, sounds like you don't have this problem then. Mostly this is an issue with motor noise from it. It actually isn't that weak I guess, just VERY loud and like it's having trouble getting revved up or something, hard to explain but the youtube videos demonstrate it.