If it's not the hard drive and not the speakers, chances are it's audible capacitor resonance, which occurs depending on how much voltage is being sent to the processor.
On my HP Pavilion DV6000Z (AMD-based), it resonates when on High Performance mode. Drop it to Power Saver mode and it goes away.
Resonance happens on most electronics. They aren't supposed to resonate at an audible pitch, but a lot of laptops do. Most people just don't have the ear to hear it or think that it's the fan or hard drive or something else.
If you can live with running on Power Saver mode (I can and do, creates less heat and less fan noise), then you should be fine. Power Saver delivers less power to the processor and changes resonance of the capacitor so it's not audible anymore.
But if you want to get it fixed, complain (politely) to HP. If it's under warranty, keep sending it in until they fix the problem. If you send it in more than 3 times and they don't offer you a replacement upgraded laptop, ask them for one. It's HP's policy to replace your notebook after 3 repairs.
If you feel you don't have much success with HP through their normal channels, email (make a formal corporate complaint to) their CEO at email@example.com
. But make sure you give their Indian tech support their chance.
My story: I had the same problem (along with external display problems). I sent my notebook in twice for repairs, and each time, they failed to fix the problem. After the second repair attempt, my notebook came back mispackaged and had scuff marks on the screen. After a while, I gave up and emailed their CEO. HP "executive customer relations" finally called and wanted to make it right. I was offered a replacement, with upgrades. I originally requested an upgrade to a TX1000 series convertible, but that didn't fly. They ended up sending me a new custom DV6000Z.
They upgraded my processor from a TL-50 to a TL-56, memory from 512MB to 1GB, from Vista Basic to Vista Premium, from standard wireless to A/B/G with Bluetooth, from a standard DVD burner to a DVD burner with Lightscribe, upgraded to a webcam, etc...
Overall, the replacement is much better. Both problems weren't completely fixed though with the replacement, telling me it's an engineering flaw, and not just a lemon. But the symptoms were less annoying. The buzzing/whine ended up being capacitors resonating. Simple fix was setting the laptop to "Power Saver". The screen problem was a result of ground-loop. That was fixed with a groundless adapter.
So simple solution: Change your power scheme and see if it goes away.