PS3 disc reading noise

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I've been hearing a noise from my PS3, and I'm wondering if it's normal. I know not all PS3's make it, as my girlfriend's 20 GB PS3 does not - but it also makes other, much louder noises at different times, so it's not like it's quieter in every way. It makes me think that there are multiple PS3 optical drive types out there, each with its own noises.

Anyway, I have a 60 GB PS3 (2006 launch one actually), and whenever a Blu-ray is playing, there is a quiet noise constantly coming from the optical drive. It can't be heard over the movie in normal viewing conditions, but can be heard if you get within 2 feet or so of the PS3. It isn't quite a pronounced clicking, but it sounds like something moving in there - there is a high frequency component to it as well, kind of like a whirring noise. Pausing the movie INSTANTLY stops the noise, so I do not think it is the disc spinning (plus I know what a spinning disc sounds like, and this is not it). If I had to guess, I'd say it's the Blu-ray laser sliding on its track. It is an extremely consistent sound - multiple clicks/whirs per second.

Has anyone else ever noticed this? It's not a problem at all if it is normal, and I'm extremely fond of my PS3 - just wanted to see if others have experienced it.