So I live in the mountains of North Carolina where it is really cold and the weather changes a lot so yesterday We got 5in of rain in one day so I had to go to work in that crap! While I was driving I hear a loud deep sound coming from somewhere under my hood when I am in 6 gear around 3,000 RPM! Like a really really deep sound not normal! I have an short ram intake and a test pipe that is all! So then today I get in my car and drive to work ( not raining anymore) and it is doing that a little bit still but not as bad! And not to mention that is 14 degrees outside when this happens... Sooo let me know what is going on!

 

With dozens of possible noises, your best bet is to post a sound clip or its just going to be a bunch of guessing.