Genius Bar Can't Replicate Problem - What Now?

Hi,

I have had an issue with my iMac where it freezes and there are consistently 3 loud beeps over and over or sometimes a very loud screeching noise. I have seen a few people saying this is a RAM issue but the problem i have is the Genius Bar couldn't replicate my problem and within 2 hours of me dropping it off they said i need to come and colelct it again.

What can i do now? When i contact Apple it always says i have to take it to the nearest Genius Bar so if they can't do anything what are my options?

Also, is there anyway perhaps static/ overheating/ interference from other devices could be the sole problem? The reason i ask is that is my iMac at home has this problem and then the other day my girlfriends brand new Macbook Pro started doing the same thing... however, in other places (e.g Genius Bar) this does not seem to happen and is very hard to replicate.

Therefore could there be an external factor influencing it?

Thanks,

Martyn