Buzzing Noise from Laptop

Sorry for this vague question, but my son is off at school and his laptop is acting up, and I can't see it or test it in any way.

He has a 3 year-old vaio laptop and he tells from time to time (usually when he is on battery power) that he hears a buzzing noise coming from side of laptop. It lasts a few seconds and then stops.

We did replace his battery a couple of weeks ago and everything seemed to be fine after the replacement, so I'm not sure these events are related. Old battery was completely incapable of holding a charge; new battery charged up and seemed to be fine. (Replacement is a new Sony battery)

Any thoughts as to what might be causing this?

Thanks,
Len

 

Perhaps check/clean the fan.