Freezing and beeping!

recently my Mac has started to freeze on me and beep very loudly in groups of three. This has not happened in the months before OS X was installed, now it seems that since August maybe once or twice a week it ffreezes, the whole PC locks up, screen still on and it beeps loudly 3 times "BEEP! BEEP! BEEP!"
then a pause, then another 3: "BEEP! BEEP! BEEP!"...and so on until I hold the rear switch down for 3 seconds to power down.
What do these beeps mean? Why have they started happening?
How can I prevent it?

3 beeps usually means the ram is bad but it could also be the ram socket as in bad motherboard best to take it in and let an authorized Apple dealer diagnose.

Thanks - I fitted an extra 4gig of ram from a reputable mfr source, so will just check it's seated firmly.

Flat Eric said:

Thanks - I fitted an extra 4gig of ram from a reputable mfr source, so will just check it's seated firmly.

Click to expand...

You can also remove one stick at a time and boot back up to determine which is not working if reseating does not work the odds of having 2 bad sticks does not usually happen.

Flat Eric said:

Thanks - I fitted an extra 4gig of ram from a reputable mfr source, so will just check it's seated firmly.

Click to expand...

Is your machine an iMac? The newer iMacs have very tight fitting memory slots. It takes considerable downward pressure to seat the modules properly.

it is October 2010 - is that new? Problem with intermittent faults like this, how long do u wait around for it to do it again? May be an evening, may be another week...

It's best to pop the modules out of the socket and re-seat them. As I stated, the late model iMacs (yours included) have very tight fitting memory slots. You have to push down on the modules with moderate force. You should hear or feel a slight click, however, the module is probably still not seated right. Keep pushing down with moderate force until you hear or feel a second click.

You can tell if they're seated by comparing them to the top slots. All four modules should line up perfectly like soldiers marching.

Also, despite buying from a reputable source it is ENTIRELY possible to simply have a bad stick of RAM. Like all other mass-manufactured products, the occasional "lemon" gets past QC and into the hands of consumers.

If following chscag's excellent advice doesn't work, I'd strongly suspect that one of the sticks is simply defective. Try to determine which one by booting up with one stick at a time till you find it, contact the seller, arrange for an exchange.