iMac crashing, loud speaker sound followed by 3 beeps

Hi,

For the past few months my iMac crashes at random. The entire system will freeze and it is then followed by a

"Crrrshhh CRSHHH CHHIIII" screeching high pitched sound from the speakers.

When I updated, this is now followed by 3 beeps.

I've searched about it being bad RAM or whatever, but I only got this Mac last October.

I don't think it registers as an error either, when I boot up again there is no mention of it.

I'm thinking about maybe upgrading to Lion if that will help? I'm not sure.

I'm also worried that it messes up the other hardware as that crunching crash sound can't be good.

I've pinpointed when it does actually seem to happen.

It seems any time I'm watching streaming content on the web, then I can usually expect a crash. I've not noticed it outside of that.

Any help much appreciated, thanks.