a fan that's sounding a warning. Fans with faulty bearings will often complain loudly when first started - then settle down somewhat as the bearing warms and the rotation speed 'settles' the vibration. The best way to check is to remove the case and listen closely - you'll be able to tell which fan is the culprit.
Don't ignore this symptom - if it's the processor fan that's failing you must replace it immediately.
An alternative cause could be a failing hard drive spindle, although this is a less likely scenario. Once again you stand a better chance of detecting whether this is the problem if the case is off.