I'm not that familiar with Bobcat excavators but I would suggest the following -
1 You can access the slew motor by removing the cab floor plate (pull out the rubber floor mat ).
2 Check the slew reduction box oil level (as has already been suggested).
3 Check to see if the bolts holding the slew motor to the reduction box are tight.
4 Check to see if the bolts holding the reduction box to the chassis are tight.
5 Drive the machine up onto blocks ( about 12ins /300mm) get underneath and with a torch check that the pinion gear shaft (from the reduction box) is not leaking and also check that there is not excessive movement on the pinion shaft bearings.
6 Get someone to position the bucket on the ground and slowly touch the bucket on the ground, if there is excessive wear on the slew ring you will see the movement, repeat this at 90 degrees, then 180 and 270 degrees. If your machine has low hours there should little or no movement on the slew bearing.
If you don't find anything wrong the my only other suggestion (without hearing the noise!) is that sometimes the slew motor brake can make a noise without causing any damage.
For Bobcat dealers in Greece go to
www.bobcat.com
Regards