Close off as many rooms as you for the next rainstorm. That way you can narrow down where in the house the smell is located.

You'll have to find it to fix it. That is how I found the problem in the old victorian we had a few years back.

What does your warranty cover? Who is responsible for fixing a problem if found? No doubt, the builder,and believe me, he is NOT going to help you find them. I suggest you find a home inspector, ask about finding leaks, water damage not visible. They have technology to help locate moisture. Ask if they have such tools. Look into the warranty to see who pays for such inspection. Also request a written report from inspector. Was the home inspected before your purchase? If not, why not?
Read your warranty, if you don't understand it, call who wrote the warranty and ask them to explain. Write it down. Then call several home inspectors, find out how they work, what tools they use. get estimate for inspector. You may need information regarding homes square footage. Please, please, follow up with this, and let us know. Good luck!

Thanks you so much for your input.
When I bout the home I was told I had a bumper to bumper warenty. I will have to sit down and read through the paper work and then follow up with some of these great ideas.
I will keep you posted as I go along.

Thanks Again!

I just toured a property with a mold expert. Not a mold remediation contractor,(who may be hoping to find a big problem to work on) but a Certified Industrial Hygienist. (CIH) He brought a moisture meter and other equipment. He checked under each sink looking for leaks. With the probe he checked a ceiling spot that looked like an old leak, but he said no moisture present there. You are going to be looking for water intrusion (roof, wall leaks, plumbing problems.)because those are spores you are smelling. a CIH can do air samples as well to see if spores are in the air.
The fact that you could smell a change after rain suggests a roof leak or a crack along a wall outside. Can you go up into the attic space with a flashlight? A home inspector who checked the house out when you bought it wouldn't have found anything necessarily because as a new home ages it settles. You can find cracks around the fireplace flashing up on the roof because that caulking cracks sometimes with a change of season. Recaulk.
Send a registered letter now to the builder detailing the fact that you gave notice earlier, and now the problem is worse after a rain.
If you installed satellite or cable and the installers made holes for their equipment on the roof or just walked over the roof--that could be the area water is entering.

We also boutht a new home a few years ago. It was a year old when we bought it. We didn't realize at that time that there was water damage at the front door. We found that when it would rain and the wind blew in from the south, it would come in through cracks around the door, seep under the door and down into the basement. Apparently, many many people in our neighborhood had this same problem. Our builder would not honor the warranty because they said the original owner should have addressed this within the first year and they hadn't.

The mildewy smell comes and goes and I would say that it kind of wafts through the house at times. I think when there is dampness then the sun beats on the door is when the smell may be worst. We are in the process of choosing a new door and soon will replace the entire thing. I am afraid of the extent of damage once they pull the old door out.

Check your door...it may be a common new home issue...it is in our area. As fast as they build these homes, it's amazing there aren't more problems!