I was driving my 525d E61 under a pouring rain and after being completely ?submersed? by a splash from a car riding in the nearby lane the car started sounding oddly. Engine noise became much louder and bass. Like a big groan from the car that was not connected to speed but to RPM. Everyone in the car noticed and seemed we had a problem with it. A couple of km later and as the weather got better sound return to normal. Has anyone experienced something like this? Any ideas what happened?

 

Just a guess here, but the brakes have the auto dry feature where they apply and then back off on their own. I'm betting you got them so wet they didn't know what to do. What's really strange is that I can hear the calipers compress and then back off when I am washing my car - without the key in it!

PS - the auto dry feature is new for 06 I think.

 

Unlikely but to be sure check your air box and see if the paper air filter looked like it got wet.