If it's been quiet in the past then it will be a blocked heatsink. and the bad news is it's a pain to get to properly! but a air duster would get the worst out.
http://www.insidemyla...avilion-dv7-notebook/
(I really wouldn't suggest trying to do it yourself, unless you have experience of stripping laptops you will break something)
If it's always been noisy then you may have found one of the many HP laptops that has really badly designed cooling in them, they underspecified the cooling and used cheap thermal pads on them rather than decent thermal compound (I had to strip my HP right down a couple of months ago and remove the pads and put proper compound in its place.
The best suggestion I can make to start with is get a can of compressed air, stick a cocktail stick in the fan vent in the bottom to stop the fan spinning and give it a good blast of air into the side vent.