I've had my mini out in the rain before, but never in a heavy torrential downpour like tonight. I left work and got on the highway, and as soon as I got up to speed and shifted into 6th...and got the rpms up to 5300rpms...I felt this strange buzzing/vibration coming from the engine (and the entire care in general). Under 5300rpms and it's gone....above 5600rpms it's gone. Just in that narrow rpm range there is this pretty heavy buzzing feeling. I shifted it into neutral while going down the highway....and it was the same as if it was in gear. So I just kept it out of that rpm range and there was no problem.

Then a few minutes later the heavy rain stopped....and I noticed that when I got back in that rpm range where the vibration was happening, it was just about gone...then it another minute or two it was completely gone.

Then...the heavy rain started back up, and so does the buzzing/vibration at 5300-5600 rpms.

So I finally get home, and pop the hood to check for any issues and notice that there is water everywhere....which makes sense because it was raining so heavy I could barely see to drive. The intercooler was all wet, and a little water make it back to the coil and firewall. I also noticed a slight bunt rubber smell.

So here is my theory...tell me if it makes sense. Could all of that heavy rain have gotten up onto the serpentine belt and started to make it slip and vibrate at that rpm range (5300-5600rpms)? Or...could it have anything to do with the exhaust being cooled down by the rain...and then after the rain stopped and everything heats back up the buzzing goes away?

It's driving me nuts....especially now that the car is in the garage, and there is no buzzing or vibration at all. Very strange. It's definitely engine rpm related...and has something to do with the rain getting something wet.

Any ideas?