My mic input (despite other stuff I have read) is - in my opinion - pretty good.
If you are getting nothing but loud noise then this isn't what people are referring to.
A couple of thing: Firstly be aware that the X3 and X3L don't supply phantom power so you cant use a condenser mic, you must be using a dynamic mic.
Try the attached file using Gearbox
(Just change .jpg to .l6t - this is because the forum software doesn't allow .l6t files to be uploaded)
This is a tone that works for me - I am not saying it is any good ;-) but it works.
Make sure that you have selected Mic as the input.
Good luck, in my opinion the Mic is Very usable.
******* EDIT *********
OK - it would appear that the Board software is now examining the content of attached files - it isn't allowing me to attach the file even if I change the extension (this was the method previously used on these boards to post tones).
I will upload the sample tone to custometone and you can download it from there!
******* EDIT 2 *********
I have uploaded the tone - it can be found here