A problem like this is very difficult to say for sure without actually experiencing the smell and to try to see where it is coming from.
A common problem we see however is with bacteria build up on the Air Conditioning evaporator in the heater housing behind the dash.
This can be cured by spraying a special antibacterial agent into the heater intake which will kill the bacteria growth and neutralize the smell. It is actually quite a common problem.
You can get products over the shelf to do this but I would recommend a good trade product from a garage (we use Wurth products) as some of the cheaper products you can buy in shops is not very good and not very effective!
I would try this in the first instance as this is a quick job and the most common problem by far.
Let me know if you need anything else
Best regards, Paul