We had a similar problem. A few things to check
First the obvious - no gone-off food or internal spillages.
Second, if you have a drainage hole at the back of the fridge, it may be blocked. Gently push something thin down the hole to clear it.
Finally, and you'll need to pull the fridge out for this and turn off the power, ther's a drip tray at the back (outside ) of the fridge. This should evaporate but sometimes it builds up and gets smelly. This may need cleaning out (this caused the problem in my case).
Hope this helps