get some compressed air and blow out your fan intakes.
run some games or a stress test to get your components to generate some heat. does the buzzing stay on full time now? if it does... then it may be a bad bearing in your blower fan. bad bearings can make a buzzing noise at high rpm.
if that isnt it... then right after your laptop is started (while its still cool and not hot) run alot of hard drive intensive tasks like copying lots of files around or running a hard drive testing program. does it make any abnormal noises more now?
my guess would be the fan.
are you techy enough to pull apart your laptop to check?