From the fault you decribe this is a fault on the cooling fan on the top of the oven thats causing this fault.
What you need to do if checking this yourself is pull the oven out and from there make sure you turn the power off.
Then remove the top cover and there you will see the cooling fan motor. Check this hasnt come loose or catching in there.
If its fine then the cooling fan motor itself will need to be replaced as it will be a noise bearing in there so all you can do is replace the cooling fan motor in here and that will cure the fault for you.
So check it out if you can as from what you describe this is going to be the cooling fan at fault here.