1.) You press the appliance onto the skin too hard. You only have to exert gentle pressure.
2.) Check the appliance for broken parts. If the appliance is damaged, replace it. If you hear loose parts rattle inside the appliance, stop using it and call the Philips Consumer Care Center in your country.
3.) Clean the appliance according to the instructions in the user manual. Especially check if any hairs or dirt are stuck in the trimming head.
 Especially when you use the appliance on wet skin with foam or gel, it is important to clean the appliance with running water after each use and to let it dry properly.
4.) After approximately 2 hours of use the disposable battery runs low and has not enough power anymore to guarantee good performance. Insert a new battery. 
Only use a disposable, non-rechargeable AA 1.5 V alkaline battery.
If the appliance still produces an abnormal sound or does not work properly, go to your dealer or take the appliance to the nearest Philips service center, or visit www.philips.com/support .

Philips Support Page